Ok, so normal trading is,
Make thread with info> Add on msn/skype/aim ect.> Make trade> Vouch> /thread.
I think it could be better, I know this is just MyBB, but I didn't know if he could implement another subdomain, which is the marketplace itself.
Here is a format I came up with
() = Checkable circle.
[ ] = Textbox.
--------------------------------------
[Username]
[What are you selling?]
[How much are you selling for?]
[Method of payment.]
() Auction? *If clicked it opens up these options*
[Starting bid.]
[Amount for autowin.]
[MiddleMan]
[Contact info]
And when someone places an offer they consider selling or buying for, there could be a button that sends a message to the chosen Middleman, and when the deal is done, it makes both members vouch, and closes the thread/deal.
Just a basic idea of what I think could help with scammers.
Also, the format is used for selling, it could be formatted into a buying format as well.