Read
"I'll Take That Unwanted Timeshare Off Your Hands-- For a Price" by Andrea Hackman, February 3, 2006.
Do not do business with any timeshare resale company that charges any sort of upfront fee. It doesn't matter if the fee is called a listing fee, an appraisal fee, an advertising fee, or a service fee. It doesn't matter if they claim to have a buyer already lined up for you, or if they claim they want to buy your timeshare for their own rental portfolio. At best, an upfront fee is unnecessary. At worst, it's a complete fraud.
If you want to sell a timeshare like Vistana, work with an established timeshare resale company with a good reputation, such as The Timeshare Store, the sponsor of this board. (I'm not affiliated with The Timeshare Store, except as a satisfied past customer.) Another option is to sell it yourself through eBay or through private ads, but you would have to educate yourself about escrow, title, legal issues, how to get the best price, and how not to get cheated by people responding to your ad.
Go to The Timeshare Store website to see the asking price for Vistana weeks. Keep in mind that the asking price does not necessarily equal the selling price. Many sellers ask too much, and eventually lower their asking price or accept a lower offer. Also, keep in mind that the seller pays a commission to the timeshare resale company and the buyer pays closing fees.