halocline
Contributor
There are no legal requirements for a visual inspection, but there are industry standards. You can't just make up your own standards. In a lawsuit, the question will be whether your inspection followed industry standards.
Actually dive shops do make up their own standards with regards to filling tanks. Yes, the annual VIP is an accepted industry standard, but any shop can peek in a tank, say "OK" and put their sticker on it and they have conformed to industry standards. There is no certifying agency that requires tank inspectors to be certified, or even trained, to do this. There would be nothing illegal at all about buying your own stickers, doing your own inspection, and then taking the tank to get filled at a shop. The PSI courses are simply an attempt to train visual inspectors (which is good) and make money doing it (which there's nothing wrong about).
Since you are concerned with the liability issue, I would consult an attorney about it, but I would bet any one of them could easily poke huge holes in the PSI training courses. IOW, I don't think being "certified" by PSI is any protection from liability or negligence in this issue. But, I could be wrong. Basically, if someone thinks you had anything to do with their injury or loss, they can sue you. And they might win regardless of what actually happened.
Aside from the liability, shops have good reason to want to make sure that the tanks they fill are safe; actually this is one of the few truly dangerous situations in scuba. That's why it's one of the very few things in scuba that IS government regulated, with the 5 year hydro stamp and licensed inspectors. That's supposed to be enough to make sure tanks are safe to fill. And it seems to be, considering that catastrophic tank ruptures during filling with air (not O2) for tanks in current hydro are basically unheard of.