The guy is full of it! DON'T buy that cylinder. Of course if you put an ad looking for a cylinder, every crooked weasel will come out of the woodwork offering to sell you a cylinder that was supposed to have been returned. As far as I know, Airgas does not sell cylinders that size. They make their money leasing the larger cylinders. If you buy any cylinder ask to see a copy of the receipt and/or where they had it exchanged. There are few places that sell cylinders but they are mostly smaller independant distributors. Their cylinders would have their sticker on them and possible their name on the neck ring or no name at all on the neck ring. If you take a cylinder in that wasn't a customer owned cylinder, they could confiscate it. If the cylinder is old, it may need a hydrotest to be re-certified. That's about $35 and you could wait a long time to get it back. Have seen several times where someone bought cylinders for cheap at an auction or estate sale only to find out they are lease cylinders and they threw their money away. Auctioneers should held liable but I don't think they are.
About a year ago the place I was working, at the time, got a call from a guy asking if they wanted to buy some of their acetylene cylinders. He had a sea can full of them, over 30 cylinders! Some were still full. What the company should have done is tell the guy to stop by so they could see the cylinders. Then they could have him charged with theft and/or possession of stolen property. The cylinders are quite unique in the industry. They were Air Liquide Altops with the built in regulators. Nobody else has them and Air Liquide does not sell them.