Interesting. In this situation, I would say that you (the collective you agreeing with this position that purchasing the guinea pig from that store was okay) are the blinded one. You all seem so terribly determined to prove that this ONE, LITTLE bit of good for this ONE animal is the most important point here, when in fact, this ONE LITTLE bit of good is undermining the GREATER good for MANY animals.
Sure, this ONE pig was saved from either a death by euthanasia or death by neglect. That's wonderful for that ONE pig. But what about the MANY other animals this pet store will continue to breed, sell, and neglect? Is this ONE pig's life worth the lives of the MANY that will follow?
Sometimes individual sacrifices have to be made for the GREATER GOOD of ALL animals. It's unfortunate for the individuals that have to suffer in the process, but I am willing to let one guinea pig suffer if it means that MANY other animals won't have to. It's never pleasant to be in the position of leaving an animal behind to a sad fate, but sometimes, in order to work towards the GREATER GOOD for all animals, it's the best thing to do.
I am very blinded by the daily suffering of thousands of animals being bred to death and being treated as merchandise, so I'm sorry, but I can't see much to celebrate about 1 purchase that helps perpetuate that cycle.