They do trade in Registry, mostly as bulk deposits. I'm not sure if you can get a Registry account with a resale contract; if you're interested in that you should probably contact Registry first. Vidanta resorts are on the weeks side of Registry. I posted the details on how they work there in this thread:
On March 5th, the Registry Collection (TRC) changed the program for Vidanta Grand Luxxe (GL) owners (at least at the 2 BR spa/1 BR loft level and up). If you deposit your contracted week with the Registry Collection, you can book two weeks through them. The deposit is good for 2 years. This...
tugbbs.com
You can find the exchange value for Grand Mayan at the link below by scrolling down to the Ts - for "The Grand Mayan." N.B., you won't be depositing a holiday week as these don't exist as fixed week contracts - you'd just get credit for whatever Vidanta wants to give you credit for, which will be a shoulder or low season week.
gl.theregistrycollection.com
There are better resorts to use for trading in Registry. I've heard good things about Montecristo Estates, which trades on the points side and gives enough points per week to book multiple weeks but has fairly high MFs and comes as 1/2 month intervals. The Wyndham Grand going in on Ambergris Caye in Belize trades on the weeks side and is supposed to get 1 for 1 weeks for better resorts and 2 for 1 for 3 star and below ones (whatever that means); MFs there are lower.