I think this would be legitimate and a nice addition. The issue of any physical fansubs people might have (not sure how many people really have physical fansubs any more anyway) would probably be OK as long as it was just trading (i.e. not selling fansubs).
I would personally hope we could avoid anyone selling or even trading bootlegged DVD's, just because of the issues involved with that.
I think this would be great though for a lot of physical anime and manga and game related stuff. There's definitely a lot of things people might end up with that they just don't care about, and they could post a thread with stuff they have they'd like to trade, and maybe stuff they are looking for in particular...... they could even update these threads as necessary, or just make new posts about specific items.
Then people could either make offers to buy them or offer something else in return for trade.
Could be very nice.