A few points of context:
1) I've bought from this store many times. The owner knows me by first name, he knows that I've bought many fish from him. I've had zero fish fatalities in my dealings with him, and used the exact same QT/transport method, and he shouldn't have any reason to suspect that.
2) I'd be rather ok with a similar deal. 1/2 store credit, half price of similar livestock. I get that I accept some liability, but the question here really is if my relationship with this store should be worth risking for something that was clearly their diseased livestock.
3) Most car dealers have a 24 hr return policy, and trust me, if the car wrapped itself around a telephone pole because it was faulty and a wheel fell off, you better believe I'd be having the broken shell of that thing towed to their lot and request a refund.