bit of a grey one i'm afraid, the ebayers may be doing it as a "hobby", i don't think that a guarantee has to be given in either trade or private sales, the old "sold as seen" and "buyer beware" happen's on both counts. unless stated in the bill of sale i don't think it will hold water in court if something goes wrong.
i may be wrong, the police or courts won't want to know unless it was stolen, you'll be told to take out a private case against the seller, they won't care if you've got a case or not in that instance, they'll just take you money.
better to sea trial and go right over it then take your chances.
if you bid a seller down on a boat and he agree's you have almost no chance, as you've told the seller what you think it's worth, technically you have valued it