Oddly enough, the pet stores in the town I grew up in aren't allowed to sell constrictors, so they don't offer ball pythons, but corns, kings, and milks are all up for grabs. The Petco reptile "expert" didn't understand what was wrong with that and kept asking if the ratsnakes I was trying to get rid of at the time were constrictors or venomous or not. This particular Petco also kept their baby bearded dragons on sand, were trying to sell a blue tongue skink that was missing at least half of it's toes and didn't seem able to use it's back legs at all, and insisted to my husband and myself that the clearly dead baby beardie was actually alive and had just eaten that morning. On this opposite side of the coin, the Petsmart in the same area has very knowledgeable reptile staff, even if their small animal department is completely lacking in common sense. (volunteered for the cat adoptions there for a while, and we couldn't figure out why the kittens kept getting ringworm. Come to find out the guinea pigs had it pretty bad, and the girl that took care of them kept coming to play with the cats on her breaks without washing her hands -.-)

I wouldn't say that all big chain pet stores are terrible, and I've seen a few mom and pop stores that weren't great either. I think it's all really just a matter of the buyer being able to identify which is which. Never buy an animal without asking questions.