Petco and Petsmart, but mainly Petco, are under investigation and boycott by PETA (people for the ethical treatment of animals). You can read all about it, the horror stories and how you can help, by going to
http://www.petcocruelty.com/. They even have an action pack you can order.
In our ignorant days, my husband and I purchased a guinea pig from Petsmart, Oink, and we were assured that Oink was only 4 months old. Within two weeks after our purchase of her, we discovered that she was closer to 2 years old than four months, and that she had cancer so far progressed that the tumor had lysed away most of her jaw bone on the left side. She died about 3 weeks later. We complained to Petsmart, and they paid our vet bill, but they wanted to take Oink back. We refused - our vet speculated that Petsmart had sold Oink many times, only to have her returned because of the tumor, and instead of having her treated, they just resold her.
We reported Petsmart to PETA, the Humane Society of the United States, our local SPCA, and to the Better Business Bureau (as they committed fraud by selling me a 2 year old pig, while claiming it was only 4 months old.) I don't know how much good it did, but at least our complaints are on the record.