Never had a problem here with used tanks. When I was ready to buy my first set of tanks 25 years ago I heard about having to watch out for stolen tanks etc. I went to airgas and several other local supplies and was told the same thing, they have no idea if the tanks are rented or owned. US Welding told me I could have walked around the end of the building and grabbed two of their tanks and told them they where mine and they would have just exchanged them with filled tanks. I kept exchanging those tanks for probably 15 years before I sold them to a friend who still has and exchanges them without ever having to prove that I owned them or whoever owned them before I bought them. I have heard that they wont exchange personal owned tanks and that you have to have them re-cert. They always just exchanged them with tanks they had in the back and I never had to have a tank re-cert done. I still have my small carry set from when I did service work and still the same process, I take them in and just exchange them when I need a full set.