The first time I purchased a hamster, it came from Petco. Unbeknownst to me as a first-time hamster owner, the one I picked was way too old to be able to socialize to humans well (at least two months old), and had wet-tail. He died three days later.
My sister twice purchased goldfish from Petco that also died within a week or two. I was disappointed that the staff sold me what I now know was a clearly sick hamster, and that the fish weren't hardy enough to even survive a very careful transition into a new environment. I urge you to give your business to a more knowledgeable and local pet shop.