I'm very lucky to have an excellent Petco and Petsmart in my city, both in the same shopping center, actually!
Probably being so close makes them have super competition, which makes healthier animals
I bought a Corn snake from the Petco, and a ball python from the petsmart, they are both very healthy and thriving, neither have any mites, diseases, or personality disorders(my BP did try to bite me on the first day, but that was my stupid mistake xD)
They do do a bit more cohabbing than I like, but there's enough hides and space for each of the babies to have their own spot.

I have heard some awful stuff about both chains though, and am glad mine are top-notch. Guess it depends on the employees.
My Petco used to suck, their animals were always sick and it was super expensive, so we didn't shop there. They were bought out in 2011 though, and since then, they have probably become one of the best in the country!

If I saw the snakes in such bad condition, I would call corporate, it's against their policy to sell unhealthy animals and the store could get in big trouble.