Not sure about the particular legality in CO. I would find it unlikely you would end up in a trouble over a couple cans of R12. Current price for 12 OZ cans is $20.00 each for quick sale and up to $35.00 a can if you want to sit on it for 6 months or a year. I still have about 25 little cans and 20 lbs. left in a 30 pound jug. Not sure of the concern about small cans leaking out, unless they rust through. I have been working off the 30 lb jug since 1998 when I first tapped it. I prefer to service old muscle cars with R12 rather than retrofit to 134A unless the owner is willing to replace all the rubber hoses. R12 stays in old systems longer especially if the system has original worm screw clamps on the hoses. It also functions better, delivering 4-6 degrees lower outlet temps. in old original systems. You will have no trouble finding a buyer at $20.00-$25.00 a can Good luck.
To answer the OP's question more directly, if you really want to cover your bases and have concern about selling R12, you can follow this link to the ASE self-certification page. Read a little about refrigerant, recycling, pass a little quiz and print a certificate. Just that easy.
https://www.ase.com/Tests/cfc.aspx