you can use mailgun for that. it has an api and concepts of "routes". you will be able to intercept email from seller@seller_domain.com to buyer@your_domain.com, handle it in your app (mailgun will POST to your web app), and then send from seller@your_domain.com to buyer@buyer_domain.com. really transparent. you can tweak any part of the workflow. have all conversations and all stats in your app.