Am I justified in my frustration?

I don't know. I see "rebuildable core" a little different here. Opinions vary, but MINE is on a rear end a rebuildable core "should be" one that may need every bearing and seal, but the ring and pinion as well as carrier and axles are in salvageable condition. You know. All the "hard parts". ESPECIALLY for $350. Bearings and seals are wear items. I mean, when any of us here do a complete bearing and seal replacement on one what do we call it? "I rebuilt the rear end".

But yup, you bear a BIG (perhaps half.....maybe more) responsibility for not inspecting what you bought. $350 is a lot of money to "me" and no matter who it was, I would have inspected it. It sounds to me like it was a $100 housing, axles and backing plates. Or, did you get backing plates? I didn't see any.

All "that", PLUS it's not right for an A body. You've got easily a grand worth of parts and work ahead of you. That's the part that should be on your radar. IMO, you've got a $100 piece of a rear end there.

Maybe the seller doesn't know how long it was outside. Honestly though, does it matter? What the HELL will that change at this point? Nothin.