Moisture, like sweat, and UV degrade Kevlar and most other high tech ballistic fabric, I believe. Ceramic SAPI plates probably hold up well over time, as long as they aren't abused. I don't think I would trust the Kevlar in used body armor if I had a choice. You just don't know much strength might have been lost. Unlike a used car, you can't take body armor for a test drive before you buy.

Of course, something is better than nothing, but degraded Kevlar might be worse than nothing if it gives you a false senes of security that increases your chance of getting shot.