No, just sign the title. It's the buyer who has to take it DGII.
I've sold two cars to buyers like that. No contract at all. I signed the back of the titles and they gave me the money. However, in my case, the buyers were used car dealers planning on selling it to someone else and wanted to avoid paying the taxes. Not exactly legit, but that was their problem. I'm not sure how they got around not having a contract with my signature on it(?).
Being dealers, they took care of all the paperwork for the buyers when they sold it and sent me a copy of the transfer of title. Make sure you get a transfer of title because you are still legally liable in any accident until the title has been officially transferred.
Anything is negotiable, but it is the buyer's responsiblity to have a legal contract drawn up, check that the car is not stolen, pay the taxes, and file the appropriate paperwork. Also, get a copy of his cedula and his number and address for your records(sounds like you might have that already).
I bought a car about 6 months ago and, as the buyer, I had a lawyer draw up an official contract, check to see if it was stolen, pay the taxes, and file the paperwork.