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.
Well firstly most accounts are set on default trust.
If a member with little to no trust were to leave negative feedback then that in itself wouldn't be enough to warn others as they wouldn't see the negative trust on the OP's profile unless they look at the untrusted feedback.
In terms of the abuse, I was referring to any and every member leaving negative trust on any suspicious account without allowing the OP to answer the questions first. I could be wrong but I'd think that people would be much more 'eager' to leave negative trust in such a case even though it may not be fully justified.
Again, that could be an option but if this is a feature that SMF provides then surely it would make more sense to disable self moderated threads for new members rather than just relying on the trust system. I don't see the harm in doing so but feel free to correct me