They list them on ebay all the time. Most list used vest as with no warranty.
I really can't see buying a used vest unless I personally knew it previous owner very well. As taking chances with used body armor aint my idea of where to go cheap on.
i would just spend the money on a new one if you are going to be a security guard...vests ability to stop bullets degrades over time and most used ones will no longer stop what they were rated for when they were new.