I'd say responsible. They are clearly stating that the thing will get much bigger, and does go on to suggest a pond as habitat.
I think that not much more than that should be needed. Considering that he is only "selling" these catfish to others within the "human" species, the fact that he is even suggesting a pond as potential habitat goes above and beyond the call of duty.
I can see a buyer saying "thanks for the information dude, but is headed for my fish steamer".
I assume that he is a pet owner attempting to re-home some of his pet catfish. I do appreciate that he is attempting to "screen" potential buyers of his pet(s) to give them the best possibility for a comfortable life in the future.
With that said, the potential buyer is not adopting a baby human! I don't think that the seller has the particular right to shove a microscope up potential buyers rectums to assure that they are super clean poop free!
I give the man props and a thumbs up!