Backend Engineer (NestJS + TypeScript) — Events & Ticketing Marketplace MVP

Description: We're building a new events & ticketing marketplace for the Saudi market - a web platform where people discover events and buy and sell tickets, with seller listings, secure checkout through a licensed payment provider, wallets and payouts, and an admin dashboard. Greenfield build, from zero to public launch. You'd own the backend and the money flow. You'll work with the founder/tech lead and the frontend engineer, and the APIs you design are the backbone of the product. This is an ownership role, not a ticket-taking role. You own everything behind the frontend: architecture, data model, APIs, payment flow, background processing, deployment. If you need direction on every decision, this isn't your role. What you own: - Architecture and data model for the whole platform - you design it, you defend it - REST APIs consumed by buyer, seller, and admin interfaces - The money flow: checkout through a licensed payment gateway, webhook handling, refunds, seller wallets, and payouts. This must be correct, not approximately correct - A unified account model: one user, buyer by default, seller capability activated on the same account after OTP verification — with roles and permissions that don't leak - A ticket fulfillment/replacement engine: rule-based matching against a queue, executed automatically on new inventory, with financial consequences on both sides - Referral and promo systems with reward calculation and monthly payout runs - Background processing with BullMQ/Redis — everything slow or third-party goes async - Integrations: WhatsApp Business API, SMS/OTP, email - Deployment: Docker, hosting inside Saudi Arabia, CI/CD, monitoring, backups The hard problems you're being hired for: - Concurrent purchases on limited inventory — no overselling, no lost updates - Payment webhooks that retry, duplicate, and arrive out of order — no double charges, no double payouts - Wallet balances that must reconcile exactly, always - Automated fulfillment matching that moves money between parties correctly Must have: - Experience in backend, with strong NestJS + TypeScript in production - You've built and shipped payment/wallet systems handling real money - Deep PostgreSQL: modeling, indexing, transactions, isolation levels, locking under concurrency - Redis + BullMQ (or equivalent) for queues, retries, idempotent job design - Docker, CI/CD, and comfort owning deployment and production issues - Security: authentication, authorization, secrets, protecting money endpoints Nice to have: - Marketplace or multi-party payouts (split payments, escrow-style flows) - MENA payment providers (Moyasar, HyperPay, Tap) - WhatsApp Business API - Event-driven patterns, observability, load testing Engagement: Contract, ~3 months, ~30 hrs/week, remote with a few hours overlapping Saudi time (AST). Start: August 2026. MVP is phase one; ongoing work after launch, scoped separately. To apply: - Begin your proposal with TICKETS-BE so I know you read this. - Share your GitHub and one relevant NestJS project involving payments or a marketplace (your role on it). - Briefly answer: how do you guarantee idempotency when handling payment webhooks, so a retried or duplicate event never causes a double charge, double refund, or double payout? Tell me your rate and weekly availability.