Auction Marketplace Build — Concurrent Bidding Engine, Payment Integration, Financial Ledger

Project Overview I'm building an auction and live-broadcast sales marketplace for high-value, expert-verified items. This is not a listings site with an auction plugin — it's a controlled ecosystem where bidding, expert review, payments, physical authentication, logistics and risk management have to work as one system. The specification is complete. I have a detailed product document covering business rules, user flows, state transitions, permissions, edge cases and operational processes. UI/UX is finished — mockups exist for all user-facing surfaces along with a locked design system. This is not a "help me figure out what to build" engagement. The thinking is done; it needs to be built correctly. Scope Three panels, different permission models: Buyer-facing marketplace (browse, bid, buy, order tracking, disputes) Seller panel (item submission, revision handling, sales, shipping, earnings) Internal panels: expert review workflow and admin/operations Bidding engine — the critical piece: Atomic, idempotent bid processing (double-clicks and simultaneous bids must never produce inconsistent state) Server-time authority for all bid timing and ordering; client device time is never trusted Anti-sniping: valid bids near auction close extend the timer, repeatable Real-time updates over WebSocket with reconciliation on reconnect Hidden reserve price logic Tiered card holds tied to bid limits, configurable from admin Automated post-auction resolution flows with time-boxed windows and defined outcomes Payments and finance: Payment gateway integration (marketplace / sub-merchant model — we do not hold customer funds) Immutable financial ledger: entries are never overwritten, corrections happen through reversing entries Per-order snapshot of commission and tax rates, so later rate changes never recalculate past orders Payouts with eligibility checks and multi-stage approval, negative balance handling Full and partial refunds, chargeback handling, invoicing, daily reconciliation Idempotency and outbox pattern throughout Expert review workflow: No item goes live without category expert approval Two work modes: items physically received at our facility (barcode intake, SLA timers) and remote online submissions Category-specific dynamic fields and checklists — different item categories require completely different data models Structured valuation and reserve negotiation where every offer is a stored record Second-opinion and dual-approval flows above configurable thresholds Risk engine: Hybrid rule-based and score-based evaluation Linked-account detection across payment method, identity, device fingerprint and IP Shill bidding detection based on behavioural patterns over time Tiered, auditable, appealable sanctions — no heavy action on a single signal Rule versioning and a test mode that evaluates without enforcing Logistics and authentication: Carrier API integration behind a provider-agnostic shipping layer Physical authentication workflow: intake, chain of custody, inspection, certification with QR-verifiable records, sealing Anonymity preserved between buyer and seller throughout the shipping process Live sale module: Live video integration with a real-time bidding layer running independently of the video stream Multiple lots per event, moderation, and failure recovery when connections drop Platform foundations: Role, permission and resource-level authorisation enforced on every endpoint, not just hidden in the UI State machines for every status-bearing entity — free-form status updates are not acceptable Audit logging of all critical actions with old and new values Money stored as decimal or minor units, never float; all timestamps in UTC Configurable business rules (rates, thresholds, timings) from admin — not hard-coded Integrations Payment gateway, carrier API, identity/KYC verification provider, and live video streaming. Providers are already selected; integration modules must be isolated so a provider can be replaced without rewriting core systems. What I'm looking for An established team or agency with genuine senior backend leadership. Specifically, prior experience with: Concurrency-critical systems (bidding, ticketing, trading, or similar) Payment integrations including refunds, chargebacks and reconciliation Event-driven architecture and real-time delivery I'm open on the technology stack. Tell me what you'd use and why. Non-negotiables Source code in our own repository from day one, with full IP assignment Milestone-based payments Named team composition — I want to know who is actually writing the code No undisclosed subcontracting How to apply Generic proposals will be ignored. Please include: Answer this in one paragraph: How would you handle 500 simultaneous bids on the same lot, and what happens if a payment webhook is delivered twice? Two relevant case studies — ideally transactional systems where money moved Your proposed team composition (roles, seniority, who leads architecture) Your recommended stack and the reasoning behind it A phase-by-phase estimate with your assumptions stated On budget: the figure listed is indicative. Quotes I've received so far range from €30k to €200k, which tells me the scope isn't being read consistently. I'm looking for a realistic assessment, not the lowest number. On process: the engagement begins with a paid pilot module (2–3 weeks, fixed price) before the full build is awarded. This protects both of us. Full specification and mockups shared under NDA once we're in discussion.