I've used TUG to find an owner and was very satisfied.
Once you contact each other and make an agreement, you'll deposit with them and pay the balance a month or a bit more before you're check-in date. Some owners will orally agree and some are willing to put in writing in a timeshare rental unit agreement.
After you've paid you're balance, you will be issued a guest certificate in you're name. You can then call the resort to confirm the reservation in you're name, dates, etc..
I've never rented from anyone on Ebay, so I can't help you there.
Good luck with you're search.