I bet what happened was that the OP purchased a car in person from a person who also had that car up for sale on ebaymotors and some other person had the winning bid which obligated the seller to sell them the car but they didn't have it anymore because it was already sold to the OP. The OP is wondering if they have to give the car back.
If the OP has paid for the car and has the title and everything is completed then it's their car now. It's an issue between the seller, ebay, and the mystery high bidder.
Did I get it right?