I have thought of something similar.

A site where several people can offer up trading with price and icons for the different ways that they will accept cash (wire, bank deposit, ing p2p, dwolla, etc).

It could provide tools and APIs for both seller and buyer to help them more easily trade on their own.

And of course, allow for ratings and statistics on each person's trades to help people get a better feel for who is the most trustworthy.

Though with FinCEN saying that any currency conversion requires a license, it could severely limit any US people doing trades.