If you still feel the need to resolve it properly, then I'd recommend contacting DJI support and explain the dilemma, as it appears you still have communication with the original owner who wishes to unbind it, unlike a found drone where DJI won't...
Well, I’ve never heard of this issue before and putting myself into the buyers shoes, even to me, it would sound like a scam. I myself didn’t believe the previous owner until I saw everything for myself. Also, it might be an issue with future...
When you add it to your DJI account it tells you it is already bound to another account? If so it would not be possible to bind it. Are you using the account from the previous owner?
Binding is confusing; there are a couple of methods and it's not always obvious and straightforward. This might help:
How to rebind a used DJI drone: Step-by-step guide