The resale of a timeshare is a legitimate business. Companies agree to advertise a timeshare for a fee on their websites. As with advertising, the membership doesn´t need to sell in order the fees to be legitimate. The difference between an honest timeshare resale company and a scam, is when the company falsely suggest they have a buyer waiting for their timeshare, or picture unrealistic, overoptimistic selling possibilities.
Beware of companies that will ask for money up front. It is very hard to sell. You might consider renting out your week yourself and make enough money to pay your maintenance fees. What resort do you own at?