What the buyer and seller agree too.
We call this novelty the free market.
Personally, I wouldn't spend more than 2/3rds what Walmart would charge for ammo someone was selling at a yard sale because you don't know if it got wet, etc. Caveat emptor.
Some of mine has been in storage for 30 years but I think I will keep a .22 rifle out so they can try it if they want.
I looked at a couple of boxes yesterday and it still looks like new.