$275 is a bit rich, since it looks to be in real rough shape. I'd be worth about $150 to me, assuming nothing is locked up and the pump still appears to pump air when spinning it by hand. The motor is probably 3 phase though, unless it has a commutator and brushes visible, then it's probably repulsion induction and single phase.
Not sure I'd trust the tank though. I probably wouldn't trust it without getting it hydro tested.