Buying Used Mattresses

My son just finished his first year at the university.  He lived on campus in a dorm for that first year.  He has moved back to his room here at the house just for a few weeks.  He has had a part time job for the past year and a half.  He pretty much supports himself.  We only pay for his tuition.  He is planning on moving in with two of his friends next weekend.  He bought some used bedroom furniture from a friend.  He will need a queen sized mattress for the frame.  I saw him looking in the classified section of the newspaper for a mattress.  Oh my goodness!  I told him it is not a good idea to buy used mattresses.  He doesn’t see any harm in this but I would certainly never do that.