Different strokes for different folks
While a saltwater tank of that size seems like the worst idea in history, I'm sure someone with enough time could make it work. Off the top of my head I'd say with saltwater- a very small live rock cave and maybe a snail or small hermit crab, brine shrimp, lol. Freshwater- cherry or ghost shrimp, a single betta, if not completely filled you could use it for fiddler crabs. You would have to do water changes at least once a day, and much like any other nano environment micro management is key.
I am very appalled that the advertiser did not put a caution on it that it shouldn't be a beginners showcase.