Important update: I was using Bitpay as payment processor for "Buy now" orders on the site. Unfortunately, they don't want me to use their service, I just received this email:
We are a payment gateway not intended for the use of exchanging cash or cash equivalents. The sale/exchange of currencies using our services is not permitted.
So I have to modify the site and switch to another payment system. I took all prefunded ("Buy now") orders offline for now and will put them online again as soon as the new payment system is in place. If you currently have a prefunded order you can of course request your Mastercoins to be sent back, just send me a PM.
If user A sends (prefunds) MSC, and user B sends BTC, what part of the trade requires the use of Bitpay?