Since I'm very particular about the looks of my ball pythons, if I were in Petco, and they had a female with a pattern I couldn't pass up, and the snakes looked relatively healthy, I'd probably buy one, because I'm confident enough now in my husbandry skills to know that I would be setting it up properly to thrive.
Quarantine is mandatory for me, set up in a PAM pretreated enclosure and a fecal test at the vet would be a must.
Would I buy any old ball python from Petco just because they have ball pythons for $30 and I'm anxious to get one? No.