Do the insurance thing!!
Then,
The trailer is going to be registered in his/her name until you get it changed at home anyway. Use the tag that is on it to get it home and mail it back to seller so he/she can turn it in or do what they need to with it.
If there is no tag on it, I would probably do outlaws suggestion and just go with it as long as it is insured. That's the main thing they they would be concerned about. ( insurance and bill of sale) 8)