Jasash, heed their advice. Especially the part of Tug Brian's article about paying up front.
My story. In 2006, my wife and I had decided to dump one of our timeshares. No financial distress or anything, just decided to cut what we owned from 3 to 2 units. Went online and started searching how to do this. I called one of the companies I came across and their pitch sounded so good. I even fell for the "upgraded" package bit where they would list my unit on multiple sites instead of just theirs (standard package was $499 and the upgraded package was $599). Long story short, they did what they promised. They listed my unit (along with 50 others at the same resort) for the 6 months they promised. Never even a nibble (in hindsight there wasn't even a chance of getting a nibble). I was out $599 and still had my unit. They spent probably less than $10 to list my unit and made out like bandits without breaking the law. It's 10 years later and I still kick myself for being so gullible that day. Don't let it happen to you.
Read Tug Brian's article. Read it again. Read all of the links. You will thank yourself down the road. Trust me, I've been there.