Hi John,
Yes, I have looked at MPB, but I haven't had good luck with them lately. The last thing I bought from them was a Leica S lens that was sent to me in "Excellent" condition that had a dead autofocus motor. Before that I had bought a Phase One DF+ body that was also in "Excellent" condition that was pretty rough with a badly pealing grip. Yes, they took both items back and I have had good experiences with them prior to those two, but it seems like lately their grading isn't what it used to be. I would also rather give my money to a fellow photographer rather than a purely money making venture (they have over $100 million in revenue per year). If it comes down to it I will probably buy from them, but I wanted to see if anyone had an old CCD-based digital back lying around first.
Nick