This is from memory, but I believe one of the stipulations on the TIP (temporary import permit) is that the boat won't be sold in Mexico. I don't know how much it's enforced or matters, but technically you need to transact and sale outside of Mexico. That means going off shore which is OK, but then when you return it's now your boat not the previous owners, and the TIP is no longer valid, so you are importing the boat into Mexico again for the first time under new ownership. But you won't have new registration papers yet, and you will have an old TIP still hanging around that needs to be cancelled.
I suspect a lot of people just ignore this, transact the deal, and move the boat back to the US or wherever, cancel the TIP, and move on. But to do it correctly looks to be a real hassle. At one point we had a boat in Mexico that was for sale and our plan was to return to San Diego to transact any deal, just to ensure it was clean and simple. If the boat's in Ensenada that easy.