The idea of trading on this board has popped up recently, and after discussing it with Yoyei and Richard, I think I can speak for all of us.
Straight trading (i.e., I'll send you two muskets for a cutlass) is not only permissible, it is encouraged! It would be great for us to be able to help each other out, especially since I am sure that all of us have bins of parts we are not using that would be very useful to someone else.
What is not allowed is selling parts/sets/customs etc. for profit. Yoyei graciously covers the costs of maintaining this site for all of us, and it is not fair for others to benefit financially from his investment. In addition, it is often the case that when money is exchanged, friendships and relationships can be harmed.
So, the bottom line is that as long as there is no financial motive behind the trading, all is well. If you have any specific questions, please feel free to contact Yoyei, Richard or myself. Thanks!