Case study · Prototyping & Information Architecture · 7 min read
Mercari Information Architecture Redesign
Why does a marketplace with millions of products leave users unable to find what they’re looking for?
Role
Product Designer & Researcher
Timeline
10 weeks
Outcome
Task success 55% → 83%
Problem statement
Mercari’s navigation was working against its users.
Mercari is a peer-to-peer marketplace for buying and selling used goods, but its information architecture was working against its users. With 17 top-level categories and a navigation bar that changed after login, users were overwhelmed before they could even begin browsing.
Our research goals were clear: strengthen trust, improve clarity, reduce cognitive load, and streamline the key buying and selling workflows. We evaluated both the browsing experience and the underlying information architecture to understand why users were dropping off.
Information overload
17 top-level categories created decision fatigue before users ever reached a product.
Inconsistent navigation
The top nav bar changed completely after login, breaking users’ mental map of the site and eroding trust.
Multi-layered browsing
The category hierarchy introduced unnecessary depth, creating drop-off points that prevented users from finding items efficiently.
My role
Product Designer and Researcher.
One of six teammates on a 10-week UCI MHCID project, working across the research, testing, and redesign.
Team: Andrea Cadena, Avi Lovelace, Carson Wang, Isabella Cedric, Pavithra Ramakrishnan, Trinah Maulion. Tools: Figma, Maze, UserTesting.com, Optimal Workshop.
Problem and solution
Problem: Users couldn’t find things, and it wasn’t their fault.
Overall task completion on the current site was just 55%. These were tech-literate adults who couldn’t reconcile Mercari’s labels with how they naturally think about products, and one participant completed none of their tasks.
Solution: A structure extracted from how users actually sort.
The new navigation wasn’t invented, it was extracted from how 49 users naturally grouped Mercari’s top-selling items in a card sort. It cut 17 categories down to 8, kept the navigation consistent before and after login, and lifted task success from 55% to 83%.
Research
Five methods, each answering a specific question.
Each method was chosen to answer a specific question about why users were struggling, not just whether they were.
Competitive analysis
SWOT framework, 6 competitors
A systematic SWOT analysis across eBay, Facebook Marketplace, Depop, PlayerTCG, Amazon, and TikTok Shop, grouped into generalist, niche, and market-leader tiers, to surface what the industry expected from navigation.
Qualitative testing
UserTesting.com, 3 participants
Watched users think out loud while navigating Mercari’s categories without the search bar, to understand how the confusion felt, not just where it occurred.
Tree testing
Optimal Workshop, 69 participants
Stripped away the visual design to isolate whether the structure itself was the problem. It was.
Card sorting
Optimal Workshop, 49 participants
Let 49 users group 59 of Mercari’s top-selling items however felt natural to them, instead of testing our own assumptions.
Quantitative testing
Maze, 21 participants
Validated the redesigned prototype with measurable data to confirm whether fewer categories actually improved navigation success.
Key insights
What the data revealed.
The numbers told one story. The user quotes told another. Together, they painted a clear picture of why Mercari’s navigation was failing its users.
Users couldn’t find things, and it wasn’t their fault
55% · task completion on the current site
Across 3 qualitative test participants asked to browse without the search bar, overall task completion was just 55%. One user completed 0 out of 3 tasks. The category structure, not user ability, was the obstacle.
“The website just looks like there’s so much going on. It’s kind of, it’s overwhelming me.” — Participant, completed 0 of 3 tasks
Category labels don’t match how users think
1 of 69 · users matched Mercari’s path for backpacks
Tree testing across 69 participants and 10 tasks showed a 52% overall success rate, dropping as low as 1% on individual tasks. Only 1 participant followed Mercari’s path through Women’s Handbags. The other 90% went straight to Handbags, a path requiring one fewer level of navigation. K-Pop photocards scored 13% success, since users don’t associate K-Pop with Toys.
High confidence in the wrong direction is just as damaging as confusion
78% → 23% · directness vs. success on the gaming controller task
For the Nintendo Switch controller task, 78% of users navigated directly to Gaming, Accessories. They were confident and decisive, but Accessories is too broad a label for hardware, and only 23% landed in the right place. Users don’t know they’re lost.
Mercari had 17 categories. Users needed 7.
17 → 8 · categories after the card sort
49 users sorted 59 of Mercari’s top-selling items. 63% of them used a median of just 7 categories to organize everything, compared to Mercari’s existing 17. Users organically created an 8th category, Books & Toys, revealing a gap in our original proposed structure.
Trust breaks when the nav disappears after login
6 · competitors analyzed across 3 tiers
Every competitor kept a stable nav before and after login. Mercari didn’t. Trust and navigation consistency aren’t differentiators, they’re table stakes.
“Platforms like eBay and Mercari both rely on third-party sellers, which makes trust and rating systems critical to the core transaction.” — Competitive analysis findings
Prototypes
Every decision traced back to a finding.
Every design decision was traceable back to a specific finding. Nothing was added on instinct. The new navigation wasn’t invented, it was extracted from how 49 users naturally grouped Mercari’s top-selling items in the card sort.
The new categories
Clothing & Accessories
Electronics & Gaming
Home & Lifestyle
Hobbies & Collectibles
Sports & Outdoors
Beauty & Wellness
Office Supplies
Books & Toys
Design system
A shared design guide maintained brand consistency across all prototype iterations. Primary typeface: Poppins.
Royal Violet
#7400B9, Primary
Majorelle Blue
#5356EE, Primary
Twilight Indigo
#252B60, Primary
Harvest Orange
#F47900, Accent
Carbon Black
#222222, Accent
Ghost White
#F2F3FE, Neutral
Full interactive prototype. Click through it below.
Results
Did it work?
Task success rate, current site to redesigned prototype
CSAT score, avg. rating 5.8 of 7
Categories, less upfront choice for more confident navigation
What the redesign solved
Task completion jumped from 55% to 83%, users could actually find what they were looking for.
Fewer categories upfront reduced decision fatigue and built confidence before anyone clicked anything.
Category labels now match how users naturally think, validated by the card sort similarity matrix.
Consistent navigation before and after login eliminated a core trust breakdown.
Trust signals (ratings, pricing, buyer protection) surfaced at the point of purchase decision.
Recommended next steps
Short term
Standardize the navigation bar before and after login, eliminating the inconsistency that breaks trust immediately.
Medium term
Ship the reduced 8-category structure. Fewer top-level choices means users scan faster and commit sooner.
Long term
Add a keyword search bar within category selection.
What I Learned
Mercari taught me that a navigation problem is rarely a visual problem. Stripping the design away in tree testing showed the structure itself was failing, and letting 49 people sort 59 items showed how far our own assumptions were from how shoppers actually think. Every decision in the redesign traced back to a finding, which made it far easier to explain and defend the design to a team of six.
It also changed how I think about confidence. On the gaming controller task, 78% of users went straight to the same place and only 23% were right, a reminder that users who feel sure aren’t necessarily succeeding, so success has to be measured, not read off how decisive people look. If I extended this work, I’d test the keyword search inside category selection and run the new structure with a wider mix of shoppers, including sellers, not just buyers.
Next step
Let’s work together.
I’m looking for my next product design role. If this work is the kind you need, I’d love to talk.