[scheides]

A few points, all IMHO:
-used parts, as-is
-if the 7-bolt was in good running condition, there is zero reason to put a 6-bolt in....sounds fishy.
-you offered to pick it back up to get it inspected, basically admitting there could be a problem with it
-it's $200, not $1000 (i.e. not a huge sum of cash).

Either give mark the money back or tell him tough luck (hopefully he doesn't return the motor through your front window). Based on your FS ad though advertising it as running condition, I would recommend just returning the cash and taking the motor back. Sell it to someone else or fix it or whatever you want to do, but starting a bunch of drama over a $200 motor is pretty much at the top of my list of lame. Mark has been wrenching on these cars for years and years and I trust his word, but here's the thing--he told me about this last week before we all went out for Kracka's bday...he said about two sentances about it to me and that was it.