I have personally seen only one "Mexican trade-in" agreement for myself. While the "trade-in" was indeed acknowledged in writing, what was conspicuously absent was any defined
time frame for transfer performance and completion. That particular "trade-in" episode also occurred in Cancun, but involved an entity with a different name.
That particular Mexican purchase, I seem to recall, involved a purchase from what was then Grupo Mayan (...since rebranded as Vida?, not sure)
In that instance, the three "traded-in" timeshare weeks (which were in the U.S., all at the same coastal FL resort) were still in the same individual's name 2+ years later.
Although the three consecutive Snowbird weeks in coastal FL had inherent resale value, it was clear that little or no effort had been made to re-sell the "traded-in" weeks or record a new deed in the U.S. in a new name (...so how much effort would have been exerted if the weeks had instead been worthless "dog" weeks in Timbuktu?).
The U.S. resort had begun hounding the owner for two (and by then, approaching three) years of unpaid maintenance fees, which is how he belatedly learned to his great dismay that his "trade-ins" were in fact still very clearly in
his name here in the U.S. --- two plus years later --- leaving him the owner of record in the eyes of the law.