Where do you live? Different counties have different laws. Ca law says you may collect rattlesnakes, but you cannot keep them in LA county, for example.
As for selling, you may not sell any native snakes. The only exceptions are cal kings, gophers, and rosy boas, so no native venomous may be sold or transported out.
You cannot buy native rattlesnakes, as there are laws on importing native wildlife into california. There may be a permit to obtain, Ive never looked into it.
You need a permit to keep exotics. It requires 2 years full time experience with a professional, like at a zoo. There is also an annual fee and some other stuff. If you go to dfg.ca.gov they have a phone number to call for restricted species permits. Im sure you can get info from there. I hear its very difficult to obtain the permit.