True, some garden centers that sell fish shouldn't, but I know of a couple that do a very good job of it. They have educated themselves and their employees to properly care for ponds and fish.
Buying fish is not like buying milk. The buyer should educate themselves and know how to identify a potential problem. You can just as easily buy a sick fish from a dealer as you can a garden center.
My advice for buying Koi and goldfish is to build a relationship with one or two reputable dealers or garden centers that you are comfortable with. If you are new to the hobby then ask someone who has been buying fish for awhile. But be warned, you will find that we all have very different opinions.