A few plates have been DRMO'ed and then get sold thereafter, whcih would make them legal to own.
Most of the stuff you see on Ebay is bringback 'oh sh*t, didn't know I still had this, might as well get some bucks for it" stuff. Pretty much anyone who has moved aroudn int eh military can verify you find all sorts of weird crap years later in your closet, garage, and basement.
Plates can get cracks and fractures to them, the only way to know is X-ray checking.
I would walk away from any plates on Ebay. If you are going to spend money to potentially save your life, spend once, spend wisely.