well here's a few things about buying used ammo.
1. you don't know how it was stored. if it's kept dry and sealed in an airtight container, ammo will last for several decades. I have cracked open ammo that was sealed in spam cans in WWII and it still goes bang. however when subjected to moisture, ammo gets corroded, the brass sweats and the powder and priming compound goes bad in years rather than decades.
2. unless you are an ammo collector, odds are you don't know what year a box of ammo was made. I have sellier and bellot in gray and pink boxes, remington that's in yellow boxes, winchester in orange boxes, etc etc. that stuff is decades old and none of it was sealed in spam cans. luckily it was stored in a semi arid location because it still goes bang and there's no corrosion...yet.
3. if someone is selling ammo they are doing it for one of 3 reasons:either they no longer have a gun chambered in it, the ammo is old and they are replacing it with new ammo, or the ammo has issues and they are replacing it with something more reliable/potent/accurate, ETC. if it's the latter 2 reasons a person should feel perfectly justified in haggling over price because there is a possibility they would be getting stiffed by purchasing ammo of dubious nature. if it's the first then you can't really fault a guy for trying to get the best price for something the seller has no use for in the first place and has no reason to hang onto.
I've been on both sides of the fence and often times I am the one being screwed because I gave them the benefit of the doubt and didn't haggle and ended up paying too much for junk ammo or I've sold ammo for way less than I wanted because I had no use for it.
while we're on the subject. a couple weeks back I was selling a bunch of milsurp ammo. I was not asking outrageous price for it, just giving myself wiggle room because everybody inevitably haggles. of course someone did and I sold it for $70 less than I was asking and about $20 less than I really wanted. well a bunch of it was plastic training ammunition and the person buying didn't do his research on what I was selling. a few hours after I made the deal he contacted me pretty P.O.ed because he thought he was getting combat grade ammunition for 1/4 it's actual value. he still got a good deal on a bunch of ammo that still goes bang, but he thought he was pulling a fast one and got burned.