Built a profit checker app for resellers that solves the "should I buy this?" problem

Hey IH! 👋

I'm the solo dev behind Underpriced.app – an AI profit analyzer built specifically for resellers, thrift store flippers, and online arbitrage sellers.

The Problem I Was Solving

I was spending 10-15 minutes per item at thrift stores and yard sales, juggling:
• Google Lens to identify items
• ChatGPT to assess condition and authenticity
• eBay sold listings to check market value
• Manual spreadsheet calculations for eBay's 13.25% + $0.30 fees, PayPal fees, shipping costs, etc.

By the time I'd researched one vintage camera or designer jacket, I'd wasted a quarter of my sourcing time. I needed something that could tell me in 30 seconds: "This $15 item will net you $87 after all fees and shipping. BUY IT."

What Underpriced Does

Upload a photo or screenshot → Get instant analysis:
✅ Deal Score (0-100) with tier rating (HOT DEAL / GOOD DEAL / FAIR / PASS)
✅ Net profit after platform fees – eBay (13.25% + $0.30), Poshmark (20%), Mercari (12.9%), Facebook Marketplace
✅ Profit margin % and ROI calculations
✅ Time-to-sell estimate + demand level (High/Medium/Low)
✅ Best platforms to list on for that specific item
✅ Authentication red flags and inspection checklist
✅ eBay comps (real sold listings to verify AI valuations)
✅ Chrome Extension for one-click analysis on Facebook Marketplace, eBay, Craigslist

Bonus Features (Premium Tiers)
• Flip Tracker – Track inventory from purchase → listing → sale with profit/loss analytics (supports up to 2,000 items on Ultra plan)
• Fake Check – Authenticity verification for designer items
• Negotiator – AI-generated negotiation scripts
• Help Me Sell – Listing titles, descriptions, pricing strategies
• Free Reseller Tools – Fee calculators, shipping cost estimators, profit margin calculators, ROI tools (40+ tools total)

The Core Insight

Resellers don't need another generic "price checker." They need a flip-focused profit calculator that factors in:
• Their actual purchase cost
• Platform-specific fees (eBay vs Poshmark vs Mercari)
• Shipping estimates (USPS Ground vs Priority)
• Time investment (items that sit for 6 months kill cash flow)
Speed beats perfection. A 30-second analysis with 85% accuracy is infinitely more valuable than a 10-minute "perfect" research session.

Tech Stack (What We're Actually Using)

Frontend:
• React 18 + Vite (fast builds, HMR)
• TailwindCSS for UI
• React Router for navigation
• Firebase Authentication (Google OAuth, Email Magic Links)

AI & Backend:
• Google Gemini Flash for image analysis and deal scoring (vision model)
• Vercel Serverless Functions (Node.js) for API
• Firebase Firestore for user data, subscriptions, and flip tracker inventory
• Upstash Redis for rate limiting (prevents abuse)

Integrations:
• eBay Finding API for sold comps (real market data)
• Stripe for subscriptions
• Brevo (formerly SendinBlue) for transactional emails

Extensions & PWA:
• Chrome Extension (Manifest V3) for one-click marketplace analysis
• Progressive Web App (installable on mobile)

Infrastructure:
• Hosted on Vercel (frontend + API)
• Firebase Hosting for email callback pages

Monetization

Freemium SaaS Model:
• Free: 10 analyses to start, 5 flip tracker items
• Reseller ($5.99/mo): 200 analyses/month, 100 tracker items, all AI tools (Fake Check, Negotiator, Help Me Sell, eBay Comps)
• Pro ($9.99/mo): 500 analyses/month, 250 tracker items, priority support
• Ultra ($17.99/mo): 1,000 analyses/month, 2,000 tracker items (for volume resellers)

First-month intro pricing: Reseller ($2.99), Pro ($4.99), Ultra ($8.99) to lower signup friction.

Unit economics work because active resellers analyze dozens of items daily. One good flip ($80 profit on a $12 item) pays for 12+ months of the subscription.

What I'm Learning About Building for Resellers

1. Speed > Perfection – Resellers want a verdict in 30 seconds, not a research paper. Even if the AI is 80% accurate, it beats spending 10 minutes per item.

2. ROI is everything – They'll pay if one flip covers the sub. A $6/mo tool that finds one $100 profit flip is a no-brainer.

3. Platform fees are COMPLEX – Getting these right builds trust:
   • eBay: 13.25% + $0.30 (some categories like sneakers are 12.9%)
   • Poshmark: 20% (flat)
   • Mercari: 12.9% + processing fees
   • Facebook Marketplace: Usually free, but shipping labels have fees

4. The "death pile" is real – Inventory that doesn't sell fast is dead capital sitting in their garage. Time-to-sell estimates are crucial for cash flow planning.

5. Trust through transparency – Showing eBay sold comps alongside AI valuations dramatically increased trust. Users can verify the AI isn't hallucinating prices.

Current Challenges

1. Balancing AI inference costs with affordable pricing
   Had to cap free tier at 10 analyses (was unlimited daily before)
   Premium features (Fake Check, Negotiator) use additional tokens
2. Sourcing reliable market data without violating ToS
   eBay Finding API is official but rate-limited
   Web scraping is risky (ToS issues, IP bans)
   Considering eBay Developers Program upgrade for higher limits
3. Building trust around AI valuations
   Added eBay comps feature (shows 3-5 real sold listings)
   Display "AI estimated" vs "eBay median price" separately
   Users trust it more when they see real data backing the AI

Questions for IH

1. For those who've built tools for niche communities (resellers, flippers, arbitrage sellers): How did you validate pricing? Did you start higher or lower than you thought?

2. Anyone successfully monetized browser extensions? Considering a standalone Chrome extension tier (e.g., $3.99/mo for extension-only access). Worth splitting the SKU or keep it bundled?

3. Best strategies for reaching thrift store flippers and garage sale resellers? They're active on Reddit (r/Flipping, r/ThriftStoreHauls) and Facebook groups but scattered across hundreds of communities. Paid ads? Content marketing? Influencer partnerships?

Open to Answering

• AI implementation for product identification & condition assessment
• Firebase + Vercel architecture for auth + serverless functions
• Chrome Extension Manifest V3 quirks & pitfalls
• Capacitor native app development (Web → Android Play Store)
• Freemium pricing psychology for reseller tools

Try it:
• Web: https://underpriced.app

• Chrome Extension: https://chrome.google.com/webstore/detail/underpriced-marketplace-d/nekmhomgakofnibfgnjdekmaaglbnmco

• Firefox Extension: https://addons.mozilla.org/en-US/firefox/addon/underpriced-reseller-checker/

Happy to dive deep on any of the above. Built this entirely solo over 6 months while learning AI integration, serverless architecture, and how resellers actually think about ROI.