hmmm, this is an interesting one...
its definitely sheridan/PMI, but the barrel seems to be shorter than a standard piranha short barrel. and considering the pump goes to the end of the barrel it could only be a PGP1 however the gun is too long for that.
so really the barrel must be a custom length, which could make it a SB piranha again, apart from the lack of back bottle which they all had.
the feed neck looks to be standard, again like a piranha
the AT its custom, none like that were ever made by sheridan/ pmi
the rear velocity adjuster looks to be palmers due to its length, and the only sheridan that came with one was a PGP2k1, which looks shorter...
the bolt though is standard.
so it seems to be:
a custom barreled,AT, 12gram run, right hand fed, sheridan....
PMI long barrels, go for 100-150 dollars....
PMI short barrels go for 80-100 dolllars.
so around that i would guess.
Put it up on MCB in their what is it section:
http://www.mcarterbrown.com/forums/what/
or in their ask the experts section and it should get awnsered with in a week.
oh and thanks for sharing, its now on my 'watching' list on ebay