If you have any grounds to rescind the transaction it must rest with the issue or ownership of the trailer. If the guy didn't own it free and clear, he couldn't sell it in the manner that he did.

You are not going to unwind the sale on the basis of the seller's alleged misrepresentations as to the unit's condition. Why?

Because you had the opportunity of an independent inspection. And none of the conditions you now complaint about could have been so latent as not to be detected by even a cursory inspection.

Next time you want to buy a used trailer, take your own and bigger bull dog.