Need a market place forum? There is no way to sell items in-game yet through auction house? Err...can we have one?

Pretty sure that theres a stock market type thing which is basically like the auction house, in the ren center.

Where's that at? I don't know what a ren center is. I only know four zones, tutorial, Desert, Canada, and Millenium City.

It's in MC.
Soon as you get there with the chopper/plane, then it's right across from you.
Hit M, zoom out, it's the big circle thing at the bottom of the map. There's a red fist near it as well.
One of the surrounding buildings is the AH, next door to it is the bank and the bank is opposite to the Arms trainer.