[TheMcs]

True QT for an LFS isn't feasible. You think prices are high in LFSs now, try them if a store has to pay to QT a fish for a month before it can be sold.
With that, the good stores are going to do everything they can to present healthy fish. It is a necessity to have a fish-only system. Moderate hypo-salinity. We ran 1.015 in the winter when turnover was higher, 1.018 in the summer when livestock hangs around a bit longer. Routine medication of the systems. Medicated feeding if necessary. Quarterly in rotation a segment is shut down and sterilized. Massive UV sterilizers running when medication wasn't present.
New shipments were acclimated and "QT'ed" in the back for 48 hours, mostly as a front-line defense to weed out obviously sick fish and those that wouldn't make it from shipping. We did place date of arrival on the ID/price sticker so people would know how long the fish had been in the store.

Regardless of what you do or don't know about anyone's water (LFS or otherwise), I would NEVER add it to your tank. Just not worth it. A fish will be fine out of the water for the time it takes to transfer from acclimation container to aquarium.

It's safe to say that nowhere along the line, from collection to your front door, has proper QT taken place. That's YOUR job.