This, some guys sell turbos without ever having them on there own car, find a good deal and flip it. It may seem fine as it spins freely and no oil in the housing but you should have had someone inspect it regardless.
Woudl you blame the seller of a car for selling you a POS if you thorughly looked it over, test drove it, had someone looked at it and the day you put it on the road the headgasket fails?
No probably not, because at the time you checked everything was fine.
I doubt thats something you can really compare it too but just trying to make a point. If he has a good seller rating chances are hes a decent dude and dident knowingly sell you a junk turb