It would seem to me, that technically u are not the owner, even tho the title was never transfered at DMV.
The fact is, u signed a release of title, delivered the title to the buyer and u were paid in full. The buyer may have some penalties to pay for failing to complete the paperwork, but his ownership of the property for which he paid is pretty clear. What documentation do u still possess that supports your claim to ownership?
In CA, this specific issue is avoided by the requirement under the law for the seller to report the sale, transfer of title and name of new owner, within 10 days of the sale. It is done with a form that is attached to the pink slip when issued by DMV. So, if u were in CA, u would also be in violation of the law.
What is the downside when 6-12 months from now, he gets out of jail in Mexico, looks for his car, then takes u to court showing the cancelled checks he paid to u and your signature and date on the pink, releasing your ownership? Seems like u have taken it upon yourself to takeover the custodial duties of his car. Sure, u have a right to compensation for doing so, but u dont have a right to use it, part it or sell it. In CA u definitely don't have a right to any of the personal property inside the car either.
U might want to discuss it with DMV as well as your buddy, and/or an atty. Doing it any way other than by the book is a gamble.
Plastic trucks suck...
Iron trucks rule...