Hi, we are really good with APIs, we have done projects using OData + OAuth2.0 which is the latest standar, same used by Twitter API, Google API etc.
As always I'll be sincere, we have never programmed the Ebay API, but we want to do it.
Based on that, to avoid you uncertainty, and to take risk away I'll offer you this:
If you contact us and give us your database or a subset of it, we can start tomorrow, without you awarding us the project, you will still have all your choices open, we will give you follow ups during the day, at this stage we will use our own Ebay API credentials to avoid messing with yours.
If we deliver and you like it, you award us with the project.
We take all the risk.
Hope it sounds fare for you.
Happy new years eve!