In a way, I am glad DJI are sticking to their rules, as it could have been mine or another user's drone and sold without permission. You also say little to nothing about eBay, or if you tried using their refund process.
You don't register the drones "under 107"...you register with the FAA....if you are flying under 107 the drone must have RID...if it has RID, it must be registered...if you occasionally use one or the other, i have to imagine that they both have...
For commercial work, you will need the FAA Part 107 remote pilot certification. You should have part 107 to get drone liability insurance. Mine is not renewing policies withOUT 107 cert #.
There is a large amount of FAA regs & info you must know...