Pet store neglect...?

Okay, quick question. My friend and i noticed that the gerbils in our local pet store are ill. Visibly ill. We asked about them and the employees brushed us off until we demanded to talk to the manager - they too brushed us off and claimed the animals where healthy.

So is there anything we can do? Such as report them, or prove they're offering sick animals for sale?

We where playing around with the idea of buying them (after obtaining assurance that they are "healthy") then getting them vetted to prove they where ill - could that be used against the store? Obviously we won't let on that we KNEW we bought ill animals.....

Just looking for ideas. We want to remain anonymous unless that absolutely won't be possible. If we can't, we'll go after the store as a whole (won't be hard, they sell puppy mill pups so i'm sure we can get a big bunch of people together to protest it - this would just be a good first start.)