Well, I've sold lots of stuff on Ebay, and bought lots of stuff. Lots of vehicles too. You friggen bought it, its yours sitting in my yard. Come and get it in the time we agreed on. When I buy something on Ebay, I go get it. There is no gray area for me. The system works on trust. Call the police if you want. Th seller should call them because your friend has abandoned his vehicle on his property. This is going to be a civil thing, not a criminal thing. The police aren't going to care. A judge will rule that your friend entered a contract by sending the money--and went out of his way to make sure the seller got it.
It's his vehicle. He needs to go get it. He didn't buy it from his mother. I hate it when I have people that think I'm raising them. It's his vehicle--tell him to spend his effort going to get it and not trying to sell it back to the former owner.
I have seen people re-auction it while it was still in the former owner's possession. He might try that. But I wouldn't go for it if I had sold it. I would be hounding the owner to come get it and not give me any bullshit. You bid, you won it's yours. That's the deal. You paid--it's REALLY yours.