I've been looking for a good chat feature for a while now, but that would be a chat room kinda type. Too bad I've gotten side tracked with other features, but don't worry, I didn't forget.
For negotiating sales specifically, I can only recommend the email way. First of all, it's private, secondly you can keep track of the transaction and thirdly, god forbid, something would get screwed up between the parties, you'd have proof of the negotiated terms.