Hi and welcome to TUG!
Have you done any research to see what your timeshare is actually selling for on the resale market? Unfortunately, most timeshares are selling for 0-10% of original retail on the resale market.
To find what yours is selling for:
Go to ebay -
http://www.ebay.com/sch/items/Timeshares-for-Sale_?_catref=1&_sacat=15897
Search for your resort by exact name
From the menu on the left click on "completed auctions"
That will tell you if any have sold recently on ebay and the selling price.
There are definitely some reputable brokers out there, but most of them charge a commission of $1,000+, so unless you own a timeshare worth thousands of dollars, it's more cost effective to sell it yourself.
Here is an article to get you started:
http://www.tugbbs.com/forums/showthread.php?t=44
If you find your timeshare has no resale value, you may want to consider giving it away:
http://www.tugbbs.com/forums/showthread.php?t=132509
What ever you do, NEVER, EVER pay an upfront fee, no matter what they call it! REPUTABLE Brokers always charge a commission AFTER the sale!