Help on messaging system

Hi guys,
I’m struggling to find a system to put the users (not necessarily registered) of my WP website directly in contact with the people offering services on my same website.

I would like to have something like the buy&sell announcement websites: buyer contacts the seller, seller receives the email but with a different sender’s address, then he answers, and buyer also receives the answer with a different sender’s address. The reason, of course, is that I collect a commission from the services sold through my website, and I don’t want sellers and buyers interact outside of my website.

Any help appreciated!