Used transmission are always a risk. You should check out any thoroughly before purchase. Its called BUYER beware not SELLER.

a $100 trans from a Z6 is AT LEAST 14 Years old. If it had a hole in the case, you should have spotted it upon inspection and declined to buy it.

As faor the internals, you said it drove fine and didnt grind but the next day it let go.

At that point the warranty is over.

Sucks but thats a hard lesson to learn. i bought a motor from a kid years ago for $100 that claimed it was perfect, and ended up being a cracked cylinder head on it and it ate coolant as soon as we started it up. he claimed it was perfect when he had it, but thats the risk you take.