Require user acceptance before allowing chat

Hi,

I'm new to drupal so of course there is a lot i don't know how to do or understand. Well, im building a site similar to craigslist (i won't go into full detail) but i'm trying to add a functionality where a person post something they want to sell, buys make an offer that only the seller can see and then if the accepts one of the offers then only can they instant message each other . Could anyone offer any advice on how this may be done or if there are modules that do this?

Thanks everyone!