See, I don't think the analogy is any different. Shops take people for a ride all the time with unnecessary repairs. If you own a car you really ought to have a passing knowledge of how it works and be equipped to figure out what a potential problem might be. That doesn't work 100% of the time, but for a lot of stuff it does.
A better analogy might be getting a dog. Say someone has a studio apartment, but the guy at the pet store says a German shepherd or a mastiff will fit just fine. The buyer has a responsibility IMO to say, "hang on, this doesn't sound right." similarly, someone buying a predaceous fish really ought to have thought, "hang on, this predator may just try and eat my fish" and passed or at least done some independent research.
In this case, the store made good because someone at the store gave terrible advice. But, in the end, this is the buyers' fault. Does the store have the obligation to not sell me clownfish to feed my lionfish because they look like nemo? No, the store has an obligation to sell product in order to stay open. The fact that we're talking about fancy fish is no different than if we were talking about feeder crickets or goldfish. They're all alive.