Legitimate sellers will have you fax an image of your drivers license and your 609 certification. The person you bought from didn't request it for a very simple reason: He didn't sell you R-12. He merely stuffed R-134a in an R-134a can and painted it R-12. You paid $30 for something that can be had at Wal-Mart for about $4.50.
As soon as I saw "Hecho in Mexico", I knew you had been ripped off. The contents of the can may not even be R-134a. There's really no telling what's in it and as one poster above suggested, it would need to be analyzed.
I would contact the EPA and turn this crook in.