Trust is based on ones own opinion.
Generally people look at someone's vouches before trading. A vouch is what people give each other after a trade has gone well. Someone with 100 vouches will generally be more trusted than someone with 10, as he/she has completed more trades so they're obviously here to trade, not to scam. However it's good to look at the quality of the trades also. So someone with say 10 vouches as all his trades were $100 each and then someone with 30 vouches but all his trades were $1 each. The guy with 10 would be more trusted in my eyes.
You can also look at ranks. Generally people with more ranks are trusted, as it shows they are involved with the forum etc. mods, Globals, admins and OMM's can all be trusted. As well as the Highly trusted rank. You can also look at donors, if someone had donated $200 to the site they are very unlikely to scam $10 or something. The other ranks are just for fun.
You must look out for Trade with Caution ranks and banned ranks as they have done something sketchy so you must use extreme caution when trading people with those ranks.
You can also look at join date and post count. Although you shouldn't base trust entirely on these, they are to be factored in.
If you need any more help just PM me or post on this thread again
(I have typed this on my iPhone and I haven't proof read because I cba and I'm tired, so if something looks weird, just try to make sense of it

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