There are legitimate companies out there listing timeshares for sale and understandably will want an upfront fee for their efforts.
I go to their site everyday looking for a timeshare.
When you deal with them as a seller you know upfront what they will charge and what they will do. You can look at their site and see other units listed there and the asking prices.
The general warning to not pay upfront fees is not good advice for a company like this in my opinion.
Even RedWeek.com charges a fee to advertise a unit for rent or sale.
I think the whole thing is going to boil down to a realistic asking price where ever you look to place your ad.
I have noticed that when I do my google search, buyatimeshare.com, redweek.com and others come up, the Classified ads on TUG do not. If you feel they will give you good exposure for your money go for it.
If you price your unit too high you won't sell it anywhere.
The unit I'm looking for has been listed there for $1,500 up to $12,000 for the exact same ownership. The one that was listed for $1,500 recently sold and the $12,000 one is still sitting there and will be for a long, long time.
I'm too cheap to even pay the low price RedWeek wants for an ad so I've use the TUG BBS, TUG Classifieds, eBay Classifieds, Craig's List, and on line forums for the resorts I've owned, so personally I would not use them unless all the other free choices didn't pan out.