I personally would be concerned if they are actually used second hand rather than new but second hand IYKWIM. The acrylic can get tiny cracks in it that cannot be cleaned & are a breeding ground for germs because of the warm milk in them which can't be cleaned out. Usually they are little scratches that look like tiny spider webs.

This is the reason it's recommended to check the bottles before every use & why they say to replace them every few months (I don't remember how often though, haven't used bottles for 4 years!).