Seeing a self moderated thread in a marketplace section in the first place should be a warning flag. On top of that, feedback can be left on the trust system to try and warn people, or you can create your own thread to warn people about a self moderated scam topic.
Could you explain what you mean by it would lead to abuse of the trust system? Flagging shady dealers seems like its the perfect use for the trust system.
But legitimate people can't protect their threads from all the garbage posts that are going to pile up in literally no time by all the illiterate sig spammers doing their daily rounds around the marketplace forum sections. If people don't use self-moderation how could they delete all the shitposters' brainfarts diluting the topic of the thread and making it hard for prospective buyers to find out relevant info by the OP? I am talking about the Services section for example because most of the threads in there are self-moderated for a reason.