Noob warning

Sorry for being a noob.

I play the game for quite some time, and I still have important questions, even though I am almost a Hero.
Today`s question: as far as I know, there are two ways of trading between players: the auction hall and the In-house trading. I still didn`t manage to figure out how trading can be performed between two players from different Houses, or between players that are not engaged in a house. What am I asking is: what is the purpose of the nordinvasion forum when it comes to trading, apart from the trade intention part? I am going to need to make some money, and I need to know what is the next step after deciding a deal on forum.
Thanks.