Well strictly speaking, slander is spoken defamation and libel is written but that's not too important here. The fact is that litigation is only expensive if it has some merit. A totally baseless suit would be tossed out of court and would therefore cost nothing. All that needs to happen here is that the site research the law on this matter and find out if they can be held liable for statements made by their members on the site. It seems ridiculous to think that even if a member was guilty of libel (meaning that the statement made was BOTH demonstrably damaging AND false) that the site would be accoutable because the libel was not perpetrated by the site. That would be crazy but even crazier still is the idea that if a member posted a statement that was not libelous because it was true that the accused vendor could successfully sue the site for anything at all. If no libel exists no one can be sued. If libel does exist the victim must sue the perpetrator. Likely all you need is a disclaimer. That's my free legal advice and it's worth every penny you paid for it I bet.
I feel a little light headed... maybe you should drive...