Help map and plan API from dataset and build API with payments

Need help planning an API from an existing dataset. The end goal for the API will be to pull data for each product, allow payment for the product then deliver the product via Amazon S3. Ability to buy more than one product at once and ability to track products bought. Will also need the ability to allow venders to be assigned to products and only allow access to products that are selected for that vendor. Would love to have a dashboard that tracked sales for the vendors and allowed the vendors to sign in and manage what products they sell and see how many views, bought products they have had via the API. What we need: Someone that can help plan the API and what the best technology to use long term. We are looking to hire a dedicated developer for the project full time/part time depending on cost, that will be paid weekly or bi weekly. If relationship is well we will love to continue working together on further projects.