When I bought my toy hauler, there was still a lien on the title certificate. The seller provided me a notarized lien release from the bank at the time cash was exchanged. I took the title certificate and that lien release to the MVD office here in New Mexico and I had no problem leaving with a clear title (having paid cash for the unit). In fact, the MVD tech almost forgot to ask for it. I provided it to her and I think she actually gave it back to me at the end of the transaction.
My rig was only a few thousand dollars, and the people lived in a $400k house, so I wasn't worried about them committing a felony for a few grand. The notarized lien release was years old, because they'd paid off the loan and not sought a title certificate without the lien.
If I were a seller and was trying to sell an encumbered piece of property, I'd be more concerned about what the buyer might think. The seller really has no obligation to clear the lien; the lien is the concern of the purchaser who takes afterward. That said, if we're talking about a lot of money, most people would use an escrow service or be concerned unless they could get a clear title certificate when cash is exchanged, and I wouldn't blame them one bit.
In the commercial world this probably wouldn't be a big deal because these things are done in the ordinary course of business all the time. In the face to face transaction world, I wouldn't be willing to risk more than I could afford to lose (as a buyer) to deal with this situation.
I suspect that there are gazillions of stolen and encumbered trailers out there since it's pretty rare for a cop to run a VIN on a trailer or even run the plates for that matter. I'm not encouraging committing crimes or anything, but I definitely worry that if my trailer was ever stolen, I'd never see it again. Fortunately I have insurance. A motor vehicle is a lot different, though; the police are running plates all the time, even if they don't pull your car over.