I'd also recommend caution. This is not a brokerage, but just an advertising company with a tricky name- GMAC Timeshare Division. Even on the GMAC Real Estate corporate site, their employees shows as licensed telemarketers, not licensed real estate agents.
http://www.gmacrealestate.com/find/office/home/Timeshare-Division-Orlando/ (click on an employees name and it will show their license designation.. Most I've seen show as LTA - Licensed Travel Agent and/or LCTS - Licensed Commercial Telephone Salesperson.... These are not licensed real estate associates......).
Many people have questioned why GMAC would sell a franchise and allow it to operate in a manner other than as a brokerage- but I haven't seen that answered.
With any timeshare marketing company, a dead giveaway that something may be wrong is the simple fact that they seem to charge whatever they can collect for the service. I've seen numerous posts on this forum all stating various amounts for the same service from the company.... An advertising company should have consistent pricing for their service. The consumer simply makes a decision if the exposure is worth the cost.
The Florida AG in the past has taken action against companies with this business model, that don't seem to have any consistency with pricing or service- but I've seen no reports about action against this group. Still, I'd recommend caution and research before agreeing to the fees.
Take time to research or to contact a resale broker familiar with your ownership who has a track record of past transactions- and try to determine the true value of what you own before making any decisions about selling. Regardless of where you advertise, you'll need to price properly to have any chance at success..
Good luck!