Backend/API Developer — Cloudflare Workers + API Integrations (Collectibles Appraisal Tool)

Backend/API Developer — Cloudflare Workers + Third-Party API Integrations (Collectibles Appraisal Tool) Category Web Development → Back-End Development (or API Development) Job Description I run a business that buys, evaluates, and resells antique and collectible items, and am building a browser-based appraisal tool powered by Claude (Anthropic's AI), backed by a Cloudflare Worker that proxies several external pricing/catalog/data APIs. The core architecture is live and partially working — I need an experienced backend developer to take ownership of the technical build so I can focus on the product/domain side. I am not a developer by trade (I have some technical literacy) — I'll act as product owner and domain expert, providing requirements, API credentials, and test cases. You'll own implementation, debugging, and technical decisions, and communicate progress in plain terms I can follow. Current state Cloudflare Worker deployed using ES Module syntax, proxying several external data providers Live routes: a catalog/reference-data API and a pricing-guide API — both functional A market-price route pulling live reference pricing from a public API, using Promise.allSettled() for parallel calls An eBay Finding API integration coded but returning 503 errors — suspect an inactive/unapproved App ID One pricing API's advanced-tier fields returning empty strings despite correct auth headers — root cause not yet confirmed (may be a subscription-tier gating issue, not a code bug) Secrets management working via Cloudflare environment bindings Scope of work Phase 1 — Stabilize existing build Diagnose and fix eBay Finding API 503 errors (App ID activation/config) Diagnose an existing pricing API's empty-field bug on advanced-tier fields Review existing Worker code for error handling, timeouts, and basic resilience (fallback behavior when an upstream API fails) Phase 2 — New integrations Additional reference/pricing-guide API A certification/authentication verification API (automate what's currently a manual lookup) Note: market price data already uses a free public API — no paid provider needed, no currency exchange API needed (all pricing sources are already USD-denominated) Phase 3 — eBay and Shopify bulk listing generation Build functionality for the tool to generate eBay-ready bulk upload files, so multiple item listings can be created on eBay in one batch instead of one at a time Build equivalent functionality for Shopify — likely via Shopify's product CSV import format or Admin API, similar pattern to the eBay export I will provide the required file formats/templates and test API access for both platforms Deliverable should let me select/export appraised items from the tool and produce files ready for both eBay's bulk upload tools and Shopify's product import Phase 4 — Ongoing General maintenance, monitoring, and iteration as the tool is used in daily business operations Possible: basic usage dashboard, caching layer to reduce API costs, rate-limit handling Phase 5 — Future (not in current scope, mentioned for context) If the MVP proves useful in my own daily operations, I may explore turning this into a multi-tenant SaaS product for other businesses in this space (auth, billing, per-client data isolation, etc.). This is not part of the current engagement, but I'm noting it because it may be relevant if you're interested in a longer-term relationship and have SaaS/multi-tenant experience. Required skills Strong JavaScript/TypeScript, specifically Cloudflare Workers (or equivalent edge/serverless experience — Workers specifically preferred) Experience integrating third-party REST APIs with varied auth schemes (API key/token pairs, OAuth 2.0) Comfortable working with sparse/inconsistent API documentation and doing your own diagnostic work (not just implementing a fully-specified spec) Experience with eBay's Seller/Trading API and/or bulk listing file formats (CSV/File Exchange format or Inventory API) is a strong plus for Phase 3 Experience with Shopify's Admin API and/or product CSV import format is a strong plus for Phase 3 Clear, proactive communicator — I need someone who flags blockers and explains tradeoffs, not just someone who executes silently Bonus: familiarity with the Anthropic API (Claude), since the front-end appraisal logic calls it What I'll provide All existing code and Cloudflare account access API credentials for the catalog/pricing providers currently in use, plus eBay (pending activation) eBay and Shopify bulk listing file formats/templates and test API access when Phase 3 begins Clear test cases and domain expertise (I can tell you exactly what output should look like for a given item/API call) Engagement structure This will start as a small, fixed-price milestone and grow from there — I'm not looking to commit to a long-term contract upfront. Milestone 1 (paid trial): Fixed price, clear deliverable, scoped below. This is the primary decision point for who I continue with. Milestone 2+: If Milestone 1 goes well, we move to additional fixed-price milestones (Phase 2 integrations, then Phase 3 listing generation), each scoped and priced individually before work starts. Part-time ongoing: Once I've found the right fit — an individual or a small team — through 2–3 successful milestones, I'll shift that person/team to an ongoing part-time arrangement (est. 5–10 hrs/week) for continued development and maintenance. I'm open to working with a small team as well as a solo freelancer, provided deliverables stay clearly scoped and one person is my main point of contact. Application requirements Please include: Relevant Cloudflare Workers (or similar edge/serverless) project examples Any experience with e-commerce or marketplace API integrations (eBay, Shopify, Etsy, etc.) Your approach to diagnosing an API returning empty fields when auth appears correct (briefly — this is one of my current live bugs) Your availability over the next 4–6 weeks