Don't let negative posts get you down. Bartering is needed more in the world, especially how the economy is now. It's a great idea. People barter everyday and don't always realize it. You will need to put a middle-man type system into the site that makes sure users always get their end of the deal though.
An example of such a system would most likely need to provide a preview of the finished content, and not allow full view until both are marked as completed on the system.
You'd also want a feedback type function sort of like Ebay's, along with some good site security. I'd recommend SSL with your own twist on captcha and login keys to prevent password crackers from hijacking accounts easily.
Oh btw, I just bartered with you giving my ideas for giving me the whole idea of a bartering website. Funny how much we use it everyday. (;