Sports card catalog with live comps

The idea

Ask a card collector what their collection is worth and they open fifteen eBay tabs, filter each to sold listings, squint past the lots and reprints, and type numbers into a spreadsheet that is stale by the weekend. The hobby moves over a billion dollars a year and its pricing workflow is manual eBay archaeology. Serious flippers pay Market Movers USD 20 a month for pro tooling, but the ordinary collector with 300 cards has nothing between that and the spreadsheet.

The wedge is doing the eBay archaeology automatically and honestly. Catalogue a card once with its set, number, parallel and grade, and a nightly job pulls real sold comps, medians them, and shows the actual listings it matched, with a confidence badge instead of false precision. Grade-aware, title-normalised matching is the entire moat; getting it visibly right is what spreadsheets and casual competitors cannot do.

Who pays

The hobbyist collector priced out of pro tools: GBP 6 a month is under one blaster box and answers a question they currently spend hours on. The free 25-card tier exists to let the portfolio chart sell the upgrade. Distribution: r/sportscards and r/baseballcards, card-collecting YouTube and TikTok (rip-and-flip creators reach exactly this buyer), and SEO on “sports card collection value tracker”. Trend charts people screenshot into group chats are the organic loop.

MVP scope

• Structured card entry plus paste-a-list parsing with confirmation
• Nightly eBay sold-comp refresh with title normalisation and grade filtering
• Confidence badges and user pinning to correct bad matches
• Portfolio value chart, movers list, per-card comp history with sold-listing links
• Free tier and two Stripe subscription tiers
• Skip for v1: photo recognition of cards, population reports, buy/sell marketplace features, and non-sport cards

Build CompShelf, a web app where sports card collectors catalogue cards and get eBay sold-listing comps automatically. Stack: Next.js (App Router), eBay Browse and Marketplace Insights APIs, Supabase (Postgres + auth + cron), Stripe.

Core flow: collector adds cards via a structured form (player, year, set, card number, parallel/variation, grader and grade or raw) or by pasting a plain-text list that gets parsed into rows for confirmation. A nightly job builds an eBay search query per card and pulls recent sold prices. The dashboard shows total collection value over time and per-card cards with last-sold price, 90-day trend, and links to the actual sold listings.

Pages: dashboard (portfolio chart plus biggest movers), collection table with inline editing, card detail (comp history chart, matched sold listings with thumbnails), add-cards flow, and settings.

Implementation details: comp quality lives or dies on query construction — build queries as "{year} {set} {player} #{number} {parallel} {grader} {grade}", then filter results by normalising titles and rejecting matches with the wrong grade or containing "lot", "reprint", "custom", or "break". Use the median of the last 5 qualifying sales, not the mean, and show a confidence badge (high/medium/low) based on match count. Let users pin the correct listing when matching fails, and learn the corrected query per card.

Pricing: free for 25 cards, GBP 6/month hobbyist tier for 1,000 cards, GBP 12/month for unlimited. Stripe subscriptions.