question about managing email communications between buyer and seller?

hi,
I have a website that connect buyers and sellers.
the buyer and seller each create an account in Asp.net membership.
After signing in, the client or seller can contact each other
by filling in textbox and then click button.
in the event handler it calls the membership table and
gets email address and forwards message.

This can work Ok - I mean the message is sent from the WEBSITE email address
so it does not reveal the buyer or seller email
BUT if I wanted keep the communication through the website
so when they login, they can see if a message is there
how would I do this simply?

thanks