Full Stack Developer / Node.js Backend Developer for a written spec of a seller API service

My marketplace has been live on Laravel and Vue for a while now and I am not changing that core. What I need written down this time is about my sellers rather than about the marketplace itself. The bigger sellers have started asking for the same thing in different words. They do not want to sit in the seller panel copying orders into their own systems by hand, and one of them asked outright whether I have an API. Right now the answer is no. I would like a written spec for a small separate service that runs next to the marketplace and gives sellers that access - reading their own orders and listings, and being told when something changes instead of checking the panel. Half of this is not the service at all. My sellers have to create their own access themselves, say where their system should be notified, and see what failed and send it again - and all of that has to live inside the seller panel they already use, which is Vue sitting on my Laravel app. So I am looking for a full stack developer who can specify both halves in one document: the service running beside the marketplace, and the screens inside it. If those two get written by two different people, I am the one left holding the gap between them. What I need the document to answer: What the new service owns and what stays in the Laravel app Which screens the seller panel needs so a seller creates their own access, changes it and switches it off without me How a seller only ever reaches their own orders and listings, with nothing else visible even by accident What happens when a seller's own system is down or slow - what is retried, what is dropped, and how they avoid processing the same order twice How I avoid breaking sellers later when something on my side changes What it costs to run, and what has to be true before I let the first seller in I have had a short spec written before for a service sitting beside the marketplace, and this one has more in it. I am not technical, so I want it written down properly before anyone builds anything, and written so a developer on a seller's side can read it too. A senior node.js backend developer view on the service half, and the saas developer angle on limits and what sellers of different sizes are allowed to do, because they are not all the same size and I do not want the biggest one setting the pace for everyone. The marketplace is already live, so the current data model, my integrations and anything about specific sellers will be shared only with the person I sign with, after NDA. I do not share any of that before contract. To apply, please share 2-3 production APIs you have built that outside customers actually consume, the front end that went with one of them, and your fixed price for this write-up.