How these sites operate. Trademarks-for-sale sites usually operate by allowing you to buy advertising, by obtaining a broker fee (usually a percentage of the sale), or by a combination of ads and fees (the eBay model). Sites that deal only in advertising (and don't take a cut) seem preferable unless the site is performing some legal overview (unlikely). Typically, these sites don't deal in the paperwork for the assignment and sale and even if they do, it might be wise to have an attorney review the docs to make sure that both the trademarks and the associated goodwill are being transferred.
What also might work. Trademark speculators may be attracted to these sites but we wonder whether anyone else is seeking them out. It's not unusual for business owners seeking a trademark to research trademark ownership at the USPTO and then solicit the owner directly. As for sellers, we wonder if they might be better off with industry-specific advertising. For example, if you have a coffee-related trademark, perhaps you would have better luck advertising in the Tea & Coffee Trade Journal or Fresh Cup. Speaking of coffee, we think we should go get another coffee to get over our post-Wolfman nightmares.
PS. By the way, we don't have the same skepticism for domain name sales websites. Most of these sites -- although you should still check them out -- seemed to have nailed down the procedures (including online escrows) that can legitimately facilitate the sale and purchase of domain names.