What terms did they offer you? Was there an upfront fee?
Here are some warnings signs with these kinds of companies:
1) Do they say that they already have a renter/buyer for your timeshare? (or an established market like people attending conventions.)
2) Are they offering to rent/sell it for far more than the market value?
3) Do they want you to pay a large up front fee?
4) Are they refusing to send you a contract until you pay their fee?
If you answer yes to any of these questions, then this is the usual scenario:
Once you pay the fee and receive the contract, you discover that the company has only promised to advertise your resort, not to rent/sell it, and they don't mention having a renter/buyer in the contract.
Then, you won't hear anything from them for a long time, and when you contact them, they will tell you that the renter/buyer backed out, but they will advertise your timeshare on their over-priced website.
Finally, when you try to get your money back, they will point out that you signed a contract, and it's only for advertising. When you try to challenge it with your credit card company, they will tell you the same thing.
When/if you decide to sell, please see the article at the top of the Buying, Selling, Renting page entitled "How to Sell Your Timeshare."
http://www.tugbbs.com/forums/showthread.php?t=44