I know that in some states you have to have special permits to sell, own, and breed any venomous reptiles and even then it's hard to police these people because they do have the permits. Or it's just illegal to own in other states. But as far as saltwater creatures I'm not entirely sure because I saw the DIY king on You Tube purchase an electric eel, was huge, and could kill him and you could see how nervous he was to release it into it's own tank. There are about 4 online sites that I've seen these little ringed guys being offered for sale but never at an LFS.
That being said I have owned a dwarf octopus and he lived about 5 months, he was fascinating to watch and interact with, and I did my homework first. There's no way to tell how old these little wonders are when you get them and they don't live but maybe over a year or so in the wild. I put him in a small glass jar with formaldehyde it was sad..
Then I saw a documentary on a man that formed a bond with an octopus shortly after mine had died. Very touching true story.
Sorry for the novel. Haven't been on R2R lately but hey to all the fellow reefers.