Backend Development Engineer Project

We are seeking a freelancer developer or Development Agency for a project, which if successful, will lead to more ongoing work. The current project is an API integration to eBay Marketplace API's. The current scope is to download and capture information in a multi tenant environment for eBay stores of various sellers, and store this data in databases on our servers. This data needs to run and update regularly, and be design to connect to our current tech stack and dashboards using micro services architecture. More general Background about us and this role: Marketplace Integrations Projects We're building out a family of independent marketplace-integration services — Selling API is the first, live today — each a self-contained .NET section with its own API, background workers, and data store, talking to a specific marketplace's API (Amazon SP-API now; Amazon Ads, Mirakl, and eBay next, same pattern repeated per marketplace). You'll do: - Build and maintain the REST API, background workers, Postgres/Mongo data layer, RabbitMQ messaging - Stand up the next marketplace sections (Ads, Mirakl, eBay) following the same tenant-agnostic, decoupled architecture - Work directly with marketplace APIs — auth, rate limits, webhooks/notifications, data sync Stack: C#/.NET, EF Core (PostgreSQL), MongoDB, Redis, .NET Aspire for local dev, GitLab CI/CD, Docker/Kubernetes. You should have: solid C#/.NET backend experience, comfort integrating third-party/marketplace APIs, and interest in owning a section from scratch rather than just extending one. To meet qualifications and be considered for this role, you must be capable to demonstrate: - Previous work relevant to these skills and needs - Communicate to an effective level of English, both oral and written - Be capable of estimating a total project cost and timeline for delivery - Be capable of estimating and agreeing to milestones - Use Upwork payment systems only for this project - Have time / bandwidth to focus on this project Any previous knowledge of eBay a big plus. Any previous knowledge of other marketplace channel API integrations also a big plus. Please note we will be vetting candidates based on their response to this brief, so please read it and demonstrate so with a starting message personalized to the brief and demonstrated to have not been written by AI. You will be joining senior members in the team capable of vetting code and testing knowledge, so this is not a role for beginners and any candidates trying to secure this job without meeting the qualifications will only waste both our time. :) In your response, please include: - Questions about the scope and depth of the project as you presently understand it. These should be questions to estimate a quote for the work and timeline (we understand these will be estimates only). - Any ideas you have based on past experiences with this project subject matter. - Reasons why you belief you are right for this role. We are seeking to secure an agency or freelancer in this position by the end of September.