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.

I’m right there with ya Rusty. To me a rebuildable core would include a good ring and pinion not rusted to ****. See the first pick. My big error was taking someone for their word because they’re in a reputable Mopar club and he’s a member on this forum. Figured he would care about reputation and representation of his club.
In the back and forth emails we shared he started listing prices of all the individual parts of an 8.75 rearend saying I should be happy I got such a good deal. Axles-$100. Yeah, if I don’t have to damage one pulling the damn thing out. Third member -$350. Okay, if I could get it off and the thing isn’t too heavily pitted on the machined surfaces. Backing plates - $50. Yes, they’re there and they might just end up being the only salvageable part.
He thinks I should feel lucky to have got such a “good” deal. I’ve got quite a few hours labor in this thing and have only got one axle freed. I’ve got $25 dollars worth of WD in it. About to have $23 worth of PB in it. I thought the ring and pinion is good to use from a rebuildable core but that’ll have to be another $150.
Most of this guys posts on here is For Sale ads. I’d be suspicious of anything this guys selling now.