I would not accept this answer. When you win an auction, you've agreed to a contract.
If it is in fact true that someone was bidding on his account without his permission, he can have fun proving that to eBay and maybe eBay will side with him, but at least make him go through the process. For all you know this is a story, and he does this regularly to people.
The story is... what... that someone logged in using his eBay credentials, and then bid on a $10,000 car and... thought he'd get away with the fraud of spending someone else's money? Or, it's a little child who didn't understand the gravity of the situation at all and was just playing around? Seems odd and specific for something someone young would do to this particular auction. Of all the cars in the world for a child to bid on, they chose yours? What's the motivation? I smell BS. I suspect "wife disagreed" or "found something else and wants out". ... which is why I suggest making him pay or go through the process with eBay. Maybe he'll have to get sworn affidavits about his supposed story, and maybe he won't be willing or able to provide those.
Maybe if this was fraud, he's obligated to officially state such and then the fraudster goes to jail for it. Same as if someone "borrows" your car and then gets a photo-radar ticket in it. You can't say "Oh, yeah, I'm not responsible, my niece didn't have my permission to drive it", without also filing a police report for theft and the niece getting charged. No wishy-washy no one is responsible kind of situation. They either stole it and they get charged or they had your permission and you have to pay the ticket and work out the consequences with her yourself, nothing in between. No "you don't have to pay the ticket because the vehicle was stolen" and also no "person who stole it doesn't get charged with theft because they had your permission". Same here. Grandson (adult? child?) bid on the car with your permission, or he committed fraud, pick one, no wishy-washy situation. He gets charged with fraud or you let him do it.
I wouldn't let him off the hook so easily. What you've got now is a contract that one party wants to un-sign. Nope, sorry.
It's a $10,000 purchase, not a game.