I bought a boat off him.
Quite a nice fellow to do business with. His son and his wife also work in the business. They all speak a little English.
Took me about a year to get all the correct documents for the boat though, but with some gentle persuasion he got it all sorted. Had to get firm, but not angry with him. He always replied with a smile and a "Si senor"!
My advice would be to agree on a price, pay about three quarters to drive the vehicle away, with the balance payable on receipt of matricula in your name.
That way he will be motivated to get the paperwork sorted for you quickly!
Try and negotiate the traspaso (transfer tax) in the deal.