If you sell someone used brass, And the buyer doesnt inspect the case and has a catastrophic failure. Are you the seller liable for anything?
Other than possibly taxes on any profit, I don't think the sell is liable for the actions of the buyer, or any third party involved.
There are some points to consider, and how much weight to give them...
First, in the US today it seems one can sue anyone for any reason. Since we seem to be past the days of lawyers not taking cases they know are not valid, or judges throwing out cases because they are, literally, stupid, nothing stops you from being sued by someone with the money to do so, and some kind of grievance, real or imagined, against you.
However, being sued for something doesn't mean there will be a judgement against you, and if you're not a "careless insensitive greedy deep pockets company" where jurors might rule against you simply to satisfy their personal sense of social justice, you're not a high profile target.
Now, lets look at the actual arguments possible for and against your liability...
Just as the injured party will claim you "knew or should have known" the brass was defective, and thereby intentionally put them at risk, and so are liable for their injury, so to are they open to the argument that THEY "knew or should have known" the brass was defective, and their failure to inspect and determine the actual condition before use would, I think, nullify their claim against the seller.
ALSO, working against their claim is the fact that they have only their word to stand on. How can they prove it was "bad brass" that YOU sold them??? I don't see how they can. Everything that was done or not done to that brass after they took physical possession of it was entirely beyond your control.
They did the brass prep, they loaded the brass, they put ammo that THEY manufactured into their gun, and fired it. The only evidence they have that you are at fault is their word against yours.
They might bring suit against you, but I don't see where it could go anywhere, and frequently false accusations result in the accuser being liable for all costs.
Every manual I've ever seen instructing how to reload/handload has the basic step of inspecting the cases to be used, and someone else's failure to follow safety rules properly is generally not your concern. OR shouldn't be, as I see it.