I think there is a national law that says that if you can prove purchase...via receipt, then you're immune from prosecution for stolen items. Otherwise anything you buy, the person who sold it to you can just say "Well, he didn't pay" and you could be charged. This has been the loophole in many stolen item sales on eBay. It isn't cool, but I'm sure it works.

It's pretty shady just paying cash with absolutely no bill of sale. It's illegal too. You can't get the car registered or insured if you don't provide a bill of sale to prove that you purchased it. I don't know about any abandoned vehicle law... by those rules, you can just lurk parking lots or alleys to see where cars are and pick them up... breaking into them is illegal. Check the carfax to see who the last registered owner was...if you can that is. If that doesn't work, go to the DMV and see who the last person to own the car is. Find them, and get them to write up a bill of sale for you. That'll be the only proof that you own the car. You don't want them showing up at your door one day just to say "I'll be taking this back now."

But I do agree, it is a really gray system regarding used cars.