What terms are they offering you?
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 asking for your credit card number before they send the contract?
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 dispute it with your credit card company, they will tell you that you only had 60 days to dispute the charge, and that it's too late to do anything.
The sad truth is that in this economy, most timeshares are selling for 0-10% of original retail, and there are many that you can't even give away.
