I don't see any reason you would be due a refund. You had the opportunity to inspect the trailer. You either did or refused to and then you accepted it as is at that point.
a contract does not have to be in writing to be enforceable. The verbal contract backed by the documents that are available would be enforceable from what I see.
the police will tell you it is a civil matter.
do not pay for anything until you are satisfied with the condition it is in. It is very difficult to prove deception, especially if you had the opportunity to inspect the merchandise. If there is no included warranty, it is basically as is.

