Try http://www.tugbbs.com/forums/
Lots of info there, the one thing you will find out is that there isn't a huge market for timeshares; and that some Marriott resports will exercise a ROFR so that any deal you make with a buyer might fall through, but at least Marriott would be taking it off your hands.

The biggest piece of advice I can give you is to NEVER pay an upfront fee to a company that promises to take the timeshare off your hand. You will have a better chance winning at 3-Card Monty than getting rid of your timeshare that way.

Good luck.