Here's an idea for a practical solution:
Make prospective buyers take a test. A committee can define minimum requirements for an ethical treatment of an animal. They should also define definite "pros" and "cons" to the keeping of different species. For instance, hamsters bite, and run on an exercise wheel late at night. Bam, there is a question or two, right there.
Next step: Employees hand out packets containing all the relevant information, and write the date large on the top. 1 week later, the prospective buyer can return and, in the store (supervised environment, but maybe the info packet should be allowed? dunno), take a multiple choice test (with several tests containing randomized question order). Pass - congratulations, you now qualify for purchase! Fail - Please try again in 1 more week. A store employee will go over wrong answers, and give more details.
This could get complicated, for instance, fish have hugely different requirements species to species, and dog breeds have different requirements as well. This is why I think subcommittees (and sub-subs, etc) should be formed. For instance, fish -> FW
-> plecos. Fish -> FW
-> tetras. Then the tetra committee can determine what info is important for tetras, and form subcommittees if the information is too vast. For example, Fish -> FW
-> tetras -> predatory tetras.
Did I explain that clearly? I'm not sure I did.