I recently started http://www.weekendhacker.net in which one of the ideas is to allow designers and developers to barter deals between them.
Currently I am referring them to http://foundrs.com/ but would potentially like to build up a barter contract for some of the projects.
What are the legal implications I should be aware of?
Many sites encourage interaction between their users and create ground rules for that interaction. Facebook, eBay, and Twitter all come to mind. Make sure that users have to take some explicit action to accept the ground rules like checking a box that states they have read the agreement. In your case, it would probably be good that they accept the agreement each time they agree to work with another user. Make sure the agreement indemnifies you in case they don't get along. I'm no lawyer, so you know the drill.