The one thing I would like to add to Ben's answer is that you will be lucky to get out of this with no proceeds. So that 30-40K paid for the timeshare maybe a total loss. If this purchase was financed with the timeshare used as collateral you may need to pay it off prior to being released.
One tactic I heard used is to offer the sales team, that sold you the timeshare, a bonus for selling yours instead of one out of inventory. Assuming their commission is typically 25% of the sales price, you might consider offering them 40% or some higher figure. Doing it this way, you will have all the slick marketing on your side probably generating the highest amount of revenue possible.
Timeshares are really bad deals. If you know this you can score some cheap vacations by attending their seminars and continuing to say no. The wife and I recently got back from a nice trip to Aruba mostly paid for with airline points, and a 2 hour timeshare tour.