I find it comical that people think there is real liability in selling scuba gear. The liability in purely on the users and some on the instructors (if they fail to follow approved training standards).
You DO NOT NEED A C-card to dive. Period, end of story. If a person wants to kill themselves scuba diving there is no law preventing it, and there is lots of case law protecting the retailers and manufactures. This is no different then Smith and Wesson, Sig, Ruger, or the local gun shop (that legally sold the firearm) being sued because some guy used the gun to commit a crime.
Now, if you want to hire a guide, hire a boat to take you to a dive site, or just about anything else, then its their insurance that requires them to ensure you have met some minimum set of qualification (ie c-card), still no law.
Air fills, that a joke, given the number of people that used scuba tanks for other purposes, ie paintball. Not to mention the only time I have been asked for a c-card was when I was getting my nitrox tanks filled. There is still no laws preventing a dive shop from selling air to anyone with a properly inspected tank. The only liability they have is providing good air and providing quality service. You can even buy a 100% O2 bottle from your local gas supplier (Roberts Oxygen for example) with nothing more than a drivers license to secure tank not the gas.