There was a discussion amongst the moderators a while back about this. It's not clearly labeled, so I'm not surprised you asked. Here's how it works (or is supposed to work!)
Want to Trade was meant to be for trading equipment and things that, like I'll trade you this conical fermentor for your Electric HERMS controller.
Sampling and Critiquing is for trading and sampling each other's beer.
I have never really paid that close of attention to the forums before, and I see that there are a TON of beer swap posts in the Want to Trade forum, (Which I actually thought was for trading beer....)
I will bring this up again with the mods and see if there is a desire to clean up the forums and rename or label the forums so that people can more easily distinguish between them.
-Homer