Usability Study · Academic · Heuristic Evaluation
Mercari Usability Evaluation
Academic Project · 2023 · UX Evaluator
Overview
Mercari is a peer-to-peer marketplace app for buying and selling secondhand goods. This academic project was a full mixed-methods usability evaluation — combining heuristic analysis with moderated user testing sessions to identify friction across the buying and listing flows.
The evaluation surfaced 14 distinct usability issues ranging from minor cosmetic problems to a severity-4 critical bug that broke a core interaction for new users.
Methodology
Phase 1 — Heuristic Evaluation
I evaluated the Mercari mobile app against Nielsen's 10 usability heuristics, walking through the core flows: account creation, product listing, search and discovery, purchasing, and messaging. Each issue was logged with the heuristic violated, the specific location in the app, and an initial severity estimate.
Phase 2 — User Testing
Moderated usability sessions were conducted with participants representative of Mercari's target demographic — casual sellers and value-focused buyers. Sessions were recorded via Zoom, with Maze used to track task completion rates and time-on-task metrics. Participants completed scenario-based tasks across listing, searching, and purchasing.
Severity Rating
Every issue was rated 1–4 using a combined frequency × impact matrix. Ratings were calibrated across evaluators to ensure consistency.
1Cosmetic — fix if time allows
2Minor — low priority fix
3Major — important to fix
4Critical — fix before launch
Key Findings
14 issues were identified across the evaluation. The most significant:
Severity 4
Listing flow crashes on image reorder
When a seller attempts to reorder photos in a listing via drag-and-drop, the app throws an unhandled error and loses all listing progress. No autosave, no recovery. 100% task failure rate in user sessions. Violates Error Prevention and Recovery heuristics.
Severity 3
Search filters reset on back navigation
Applied search filters are cleared when a user navigates back from a product detail page, forcing them to reapply. Causes repeated friction in the discovery flow. Violates User Control and Freedom.
Severity 3
Shipping cost not visible until checkout
Buyers cannot see total shipping cost until they begin the checkout flow, creating a surprise at the point of commitment. Frequently cited in user sessions as a reason to abandon. Violates Visibility of System Status.
Severity 3
Category selection requires excessive scrolling
The listing category picker presents all categories in a flat scrollable list with no search or grouping, requiring significant scroll to locate specific categories. Violates Efficiency of Use and Flexibility.
Severity 2
Offer messaging lacks context
When a buyer receives a counter-offer, the message thread doesn't show the original listing price alongside the offer, requiring users to navigate away to compare. Violates Match Between System and Real World.
Severity 2
Sold items still appear in search results
Completed listings occasionally appear in search results without a clear "sold" state on the thumbnail, leading users to open unavailable products. Violates Visibility of System Status.
Severity 1
Inconsistent icon labeling across tabs
Some bottom navigation icons include text labels while others do not, creating visual inconsistency. Cosmetic issue that doesn't affect usability but reduces perceived polish.
Results
14Issues documented
1Severity-4 critical bug
3Severity-3 major issues
100%Task fail rate on critical bug
What I Learned
- Heuristic evaluation and user testing catch different issues — the critical bug wasn't obvious from heuristic review alone, but surfaced immediately in the first user session.
- Severity ratings are only useful if they're calibrated. Individual evaluators have different thresholds — discussing and aligning before rating produces more consistent results.
- The biggest usability issues in real apps are often not UI problems — they're data and state management failures that manifest as UI confusion.
