You generally can tell by their vouches, what kind of trades they do. You have to decide to go first, MM are optional, not required. Plznate is the best MM in my opinion.
you just use a middle man, which means for example, i give him irl money and yellow+fkit, you give him white phat, then he gives you my items and gives me yours, (if you were to trade for what im offering) (note: this trade was only an Example)
dont forget people are only buying street value, as you arent physically trading them the partyhat, you're trading it for GP, so the partyhats price take away the hefty 500m+ trade price, leaves around 2.5b street for white? and 1m street is worth about what..? well $20 = 175m?