Giving a marketplace a social identity
Unique usernames and mentions for Mercari, designed to fit an identity system built for buying and selling.
Summary
Mercari is a peer-to-peer marketplace where people connect through buying, selling, and community discussions.
Mercari's display names weren't unique, so people couldn't reliably find, mention, or verify each other as the marketplace added social features. I led design end to end for unique usernames and @mentions, from the identity hierarchy to onboarding, account settings, cross-platform mentions, and an internal support tool.
Team: Lead UX designer (me), Content designer, PM, 3 engineers, 2 QA, 2 GTM
Duration: Jul 2023 - Apr 2024 (shipped in phases)
Result: +21% increase in comments
The problem: Display names weren’t built for social interactions
Mercari allowed duplicate display names, making it difficult to uniquely identify or mention other users.
Introducing usernames meant answering these 3 questions:
Identity: How do two names coexist without making people relearn who they're buying from?
Adoption: How do new and existing users claim a username inside account flows that already exist?
Interaction: How does @mention work on mobile and desktop, inside comments that already support hashtags and links?
Before: display names aren't unique and can contain emoji, which makes them hard to search or mention. After: every person also has one unique @username
The outcome: One identity system across every surface
Display name first, unique @username second, applied consistently across profiles, item pages, messages, and comments.
How might we add social identity without making Mercari feel less like a marketplace?
Decision 01 - Display name leads, the @username verifies
The challenge
Buyers recognize sellers by display name. Adding a second identifier risked clutter, or pushing aside the name people already trust.
Competitive analysis highlighted different approaches to balancing recognizable display names and unique handles, which informed our decision to keep display names as the primary identifier.
What I explored
Four hierarchy variants (inline vs. two lines, same vs. smaller username), each tested with maximum-length names on the seller card, the Messages list, and list rows.
The decision
Keep display names primary and place usernames on a secondary line.
The tradeoff
Rather than giving both identifiers equal visual weight, we prioritized recognition and readability while ensuring usernames remained discoverable.
Decision 02 - Align username creation with the account lifecycle
The challenge
My initial design collected usernames during registration, but engineering revealed that mobile accounts weren't created until SMS verification.
The decision
Move username creation to post-verification onboarding, aligning it with the existing account system.
The tradeoff
A separate setup step instead of collecting everything upfront, in exchange for a more feasible implementation.
Beyond onboarding
I also designed username entry points for existing users, account settings, and an internal management tool.
Decision 03 - Make mentions intuitive across platforms and conversations
The challenge
Mentions needed to work across devices without disrupting existing comment interactions.
Mobile
Selected a bottom-sheet suggestion list to keep user selection within the comment flow.
Desktop
Chose fixed positioning over a dynamic list to reduce visual disruption while typing.
Content integration
Designed how valid and invalid mentions coexist with hashtags and links.
How I worked
Defined identity principles collaboratively
Partnered with the content designer to establish username hierarchy, informed by my competitive analysis.
Design across the account lifecycle
Mapped identity touchpoints and refined onboarding with engineering around account creation and validation constraints.
Prioritized the core experience
Worked within a fixed launch timeline, focusing on usernames and mentions while deferring broader user search.
Impact after launch…
Following the launch of unique usernames and @mentions, Mercari observed increased engagement across the platform.
+21% total comments
Increase in overall commenting activity following launch.
+10% daily active users
Increase in overall platform activity following launch.
Together, these results indicated positive engagement trends following the introduction of usernames and mentions, establishing a foundation for further social interactions.