In my area there are a lot of folks filling transfer type containers and I have never heard of or seen any station operator ever look at or comment on containers. I have even seen guys and gals filling glass gallon jugs. The law is very clear though that all such containers have to be approved. I doubt even patrolling police bother unless it is obvious that the container is not secured or stopping for some other moving violation. I was involved in an incident several years ago; a guy had two old rusty 55 gal drums in the back of his pickup, not secured, and he was just completing filling them when I drove up to an adjacent pump. I asked the station attendendant why he was allowed to do this as 55 Gal drums are not an approved transfer container in WA unless sold and transported by a licensed dealer. His statement to me was why don't you mind your own business, I want that big 100 GL sale and I do not care as I am not the gas police. I watched the guy pull away with a jerk and both drums slammed into the tailgate and almost popped the latch and bent it up badly. I jotted down his licensce # and called the fire marshalls office. They confirmed what the guy was doing was driving a huge bomb around. They investigated, fined the guy, confiscated the drums and gas. They called me back and thanked me as they saw that if he slammed the drums back one more time they would have been rolling down the street.
Moral: No matter what you use, secure it when moving.
