Celtech,
Most quality bullet resistant vest makers put a 5 year life on them. I have been told by factory reps that the vest will not magically stop working after 5 years, but that the manufacturers (and the National Institute of Crimnial Justice Research) use that as a cut off. The care of the vest is very important to its continued usefulness. If it is exposed to a lot of heat (as in left in a car trunk for a number of months or years) or moisture (rain etc) and not dried properly, the fibers will break down and be less effective.
If you a willing to bet your life on a used vest of unknown care history or age, fine. I'm not so sure I would. If the cost of a new vest is the problem, look into one with a lower threat level, they do cost less. But realistically, the rule of thumb for threat level, is to buy one that will defend against the weapon and ammo YOU carry. A lot of cops (and others in high risk occupations) get shot with their own gun.
Another point to consider. You didn't mention your occupation, but if there is a high risk if stabbing type weapons, most vests are not a real protection. There are specially designed ones for corrections officers, etc., that increase the protection against stabbings.
Hope this helps.
