I started my own trading board a while ago, but I never finished it (yes - I was making it from scratch). It was a trading board rather than forum - you offered Pokemon up, and people accepted offers, added comments etc. In fact, I got it to a nearly-complete stage, but I just couldn't get people to it. I thought it was pretty good too. If you want, you can take a look at it
here (login with username test and password test). Feel free to post, accept, and mess around with it
My point is, you need to have something special,
and be able to get users to it.