Marketplace Design

---
name: marketplace-design
description: "Use when someone asks how a catalogue, listing grid, comparison view, detail page, or transaction bridge should help participants discover and choose in a marketplace. Produces information-architecture specifications for search, filters, listings, comparison, and the path to transaction. Use `conversion-design` for a single-product marketing page and `workspace-design` for an internal management tool. Marketplace strategy is upstream; framing and sizing the marketplace bet belongs to product engineering; implementing search, filters, transactions, or listing copy belongs elsewhere."
---

# Skill: marketplace-design

Converts the buyer journey and the listing object model into a **structural specification for a marketplace surface** — the listing card IA, the filter and facet architecture, the comparison affordances, and the transaction bridge that carries a buyer from discovery to commitment. This skill is IA and structure; it does not design individual card components (that is `interaction-design`'s component state machine) and does not derive tokens or color (that is `design-system` and `creative-direction`).

## Output rendering

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Lead with the useful outcome or next action. Use warm, non-blaming language and everyday words. Define an unfamiliar term in a few plain words before naming it; keep proper names and exact technical terms intact.
During tool work, do not narrate routine calls. Send an update only for safety, a blocker, a needed decision, a material scope change, a long wait, or an active host requirement.
When requesting input, ask only for what is needed now. Ask dependent questions one at a time; otherwise group related questions. Offer no more than three clear choices when choices help.
Shape the answer to the facts: one fact needs one sentence; related facts use prose; separate items use bullets; real sequences use numbered steps.
For prose artifacts, use descriptive headings, short resumable sections, one fact per sentence, and no repeated summary. Emphasize at most one load-bearing point per section. Group long inventories instead of truncating them.
Make the result stand alone. Do needed arithmetic, give real dates or times, and say what a file or link establishes instead of making the reader inspect it.
For code and comments, prefer obvious structure and names. Comment on intent, constraints, or trade-offs that the code cannot state clearly.
Use a table, tree, flow, or other visual only when it makes a relationship materially easier to understand.
Report the current state, not the path taken. Omit dead ends, resolved trade-offs, hedges, and advice the user did not request.
When editing maintained prose, consolidate repeated rules and navigation before adding another caveat.
Silence and brevity never reduce the work, checks, or requested coverage. Preserve depth, evidence, constraints, warnings, code, diffs, errors, and exact names, paths, and counts.
Keep verification compact: pass or fail, count, and runtime. Name a suite when it failed or when the name changes what the reader should do.
Before sending, check that the reader can act without counting, converting, opening a file, or asking what a line means.
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Table — When presenting several items that share the same fields, render a Markdown table. Cap at ~5 columns; beyond that, switch to a per-item detail list. Right-align numeric columns.

## When to invoke

Confirm all three before specifying:

1. **The surface mediates a match between two parties** — a buyer choosing a seller, a user selecting from a catalogue, a customer finding a service. If the surface sells a single product directly, use `conversion-design`.
2. **The listing object model is defined** — what attributes does each listing carry? Which attributes drive filtering? Which drive comparison? The card IA and filter architecture are downstream of this model.
3. **The dominant buyer behavior is named** — browse-first (I don't know exactly what I want, show me options) vs. search-first (I know what I want, help me find it quickly). These produce fundamentally different surface structures.

## Listing card IA

The card is the atomic unit of a marketplace. Its hierarchy determines which attribute drives the match. Structure the hierarchy in order of decision weight:

1. **Primary identifier** — the name or title that lets the buyer know they're looking at the right category of thing (product name, service title, pack name)
2. **Key attribute** — the attribute that most immediately qualifies or disqualifies the listing for this buyer (price, rating, availability, compatibility, experience level)
3. **Social proof signal** — one trust signal visible on the card (rating score + count, badge, review count, notable customer name)
4. **Secondary attributes** — supporting details that inform, not decide (category tags, secondary specs, provider name)
5. **Primary CTA** — the action that advances commitment (View, Get, Install, Book, Select)

**Card density vs. detail page:** the card carries enough to qualify; the detail page carries enough to commit. Do not put commitment-level information on the card; do not put qualification-level information only on the detail page. A buyer who must open every card to qualify listings has a card that is doing the detail page's job.

## Filter and facet architecture

Filters reduce the consideration set; they do not select. The architecture decision — chip vs. sidebar, instant vs. apply-button, single-select vs. multi-select — follows from the filter purpose:

**Taxonomy design (before filter UI):**
- Name the dimensions a buyer filters on (category, price range, rating, compatibility, availability)
- For each dimension, name the options: are they enumerable (6 categories) or continuous (price range)? Enumerable dimensions suit chips or multi-select lists; continuous dimensions suit range sliders
- Name which dimensions are mutually exclusive (a listing is in exactly one category) vs. multi-valued (a listing can have multiple tags)

**Chip vs. sidebar:**
- **Chips** (horizontal filter row above the grid): best for ≤6 high-frequency filter dimensions; all visible at once; favors mobile and browse-first behavior
- **Sidebar** (vertical filter panel left of the grid): best for >6 filter dimensions or complex taxonomies; allows nested facets and filter groups; favors search-first and high-intent behavior

**Instant vs. apply-button:**
- **Instant** (grid updates as user selects): best when the consideration set is small or filter combinations are simple; provides immediate feedback on result count
- **Apply-button** (grid updates on explicit confirmation): best when filter combinations are complex or network latency makes instant updates jarring; required when filter selections are expensive to compute

**Empty-filter vs. zero-results distinction:**
- **Empty filter** (no filter applied, all listings visible): the starting state; the grid is full
- **Zero results** (filters applied, no matching listings): a design failure if it appears without explanation and recovery. Always show: which filters produced zero results, and the closest partial-match suggestions or the "clear filters" affordance

## Comparison affordances

When buyers need to compare listings before committing, design a comparison affordance that is available before the decision to compare is made:

- A **compare checkbox** (or toggle) on the card, visible before the card is opened — comparison intent surfaces at the browse stage, not after the buyer has already invested in individual detail pages
- A **comparison panel** or **comparison page** that aligns attributes in parallel columns — each column is one listing; each row is one attribute. Attributes that differ between listings are the decision levers; attributes that are identical are de-emphasized
- A **maximum comparison set** — comparing more than 4 listings simultaneously exceeds working memory. Limit to 4; prompt to remove before adding more

## Browse-first vs. search-first routing

**Browse-first surfaces** (buyer is exploring, not searching):
- The grid IS the primary experience; search is secondary
- Filter architecture is chip-based, immediately visible
- Card density is lower (more white space, larger images, fewer attributes) to invite exploration
- Sort order defaults to "recommended" or "popular"; the buyer is not yet ready to optimize on a specific attribute

**Search-first surfaces** (buyer knows what they want):
- Search is the primary experience; the grid is the result set
- Filter architecture is sidebar-based, complex, available immediately on search
- Card density is higher (more attributes visible, smaller images) to enable rapid qualification
- Sort order defaults to "relevance" to the search query; the buyer is optimizing, not exploring

## Transaction bridge to interaction-design

The point where a buyer commits — cart, checkout, booking confirmation — transitions from marketplace IA to wizard/stepper interaction design. This transition is a design seam:

- Name the handoff point explicitly (what action triggers the transition from marketplace to transaction)
- The transaction flow is a wizard pattern (see `interaction-design`'s wizard-and-stepper family): linear steps, save-and-resume, form validation
- The marketplace context (what listing the buyer selected) must be visible throughout the transaction flow — buyers abandon when they lose sight of what they are committing to

## Canonical aesthetic reference tier (study subjects, not prescriptive tools)

For grounding creative-direction on a marketplace surface, study how these products handle listing clarity and filter architecture: Airbnb (browse-first, map-integrated, social-proof hierarchy), GitHub Marketplace (developer tool catalogue, badge-first trust signals), npm (search-first, high-density reference information). Internalize the structural philosophy — card hierarchy, filter legibility, comparison design — not the surface treatment.

## Anti-patterns to refuse

- **Card that does the detail page's job.** A card that requires the buyer to open it to qualify basic attributes has a card that is too sparse, or a detail page that is too deep. Restructure the card hierarchy.
- **Zero-results with no recovery.** An empty results page with no explanation of which filter produced no results, and no "clear filters" path, is a dead end.
- **Comparison without a maximum.** Comparison with no upper limit produces a broken layout at 6+ columns. Set the limit; enforce it in the affordance design.
- **Browse-first grid with search-first density.** A browse grid filled with small, high-density cards signals "I know what you want." If the user is exploring, lower density and more visual space reduce cognitive load.
- **Transaction flow that loses marketplace context.** A checkout that shows only the cart, not what the buyer selected or why it matched their need, increases abandonment. Keep the listing context visible.