If you have a tractor stolen, I think the burden is on the thief to prove that he purchased the tractor from you. Either bank statements, check, notarized or un-notarized receipt from you, etc.
So if you are buying a used tractor, (assuming we are talking something of significant value and not a $1200 "N" series ford), make sure you get something in writing and signed by the seller, and pay with a cashiers check.
Its in the best interest of both the buyer and seller to do a "bill of sale" and cashiers check route so there is a paper trail. The seller cannot then claim it was stolen so that protects the buyer. The seller is protected by not carrying a wad of cash to a deal, and terms of the deal are clearly spelled out in the BOS, like exact model, serial #, Attachemnts, etc. So it releases the seller of all liability and ties to the tractor (assuming they have paid off their loan.)