No single LFS can prevent ich. If you fully understand ich's complex life cycle and the proper treatments, you'd understand why pretty much every LFS has ich in its system. No LFS can quarantine/treat every one of its fish for four weeks prior to selling them. They would be quickly out of business if they do that.
The most a LFS can do is to maintain good water quality, not overstock its tanks, so the fish stay healthy and don't get heavily infested by ich (or other parasites). For us hobbyists, we have to assume that every fish we buy has some sort of parasites, and properly quarantine it and provide treatments.