Funny, but I bought my pretty small 33" inert gas, oxygen, and acetylene bottles (sort of medium size) via CL from an old guy with a forest of them in his yard. He wrote some cheesed up titles (from car title copies) for them that I figured were useless. He seemed pretty honest in that he said the dates on the bottles were important and had to be under 10 years (I think)...
I just took the 3 bottles to the nearest gas supplier who checked the test dates on them and exchanged them all for full ones w/o a problem. Like others I think the inert gas was about $40+/- a bit. I let half of it escape by finger tightening the regulator on the bottle the first time
So in theory, I own the tanks/bottles and can exchange them at least w/ that gas company.
It cost probably a bit over $100 for the bottles... I thought it worked out well.
Long ago I bought oxygen, and acetylene bottles at an auction and the gas company people said they couldn't be exchanged. I loaned the torch setup to a friend w/ the understanding that he refill the tanks if needed. They were exchanged w/o a problem. Different day, different answer...
If you can troll CL for a bit, try that...