You would think that it would be safe, but it really isn't. When you put a fish in your own quarantine tank it should remain there a good 6 - 8 weeks. That's not really bouncing it around much. As long as you give it a quarantine tank big enough ( not a 10 gallon tank) and give it plenty of hiding places, good water, and good food, it should get fairly comfortable. I like to cover the ends of the tank and front of the tank with paper to give it more privacy. I leave holes in the paper to be able to observe the fish, but not big enough so the fish sees this giant being coming at it, with no place to flee to.
A fish store employee that inadvertently uses a net in one tank that does have a sick fish in can cross contaminate the tank the day before you pick it up ( or sooner)
The tank could be connected to a central sump, that has other tanks that have sick fish in.
There are so many reasons to use a quarantine tank, and none ( for fish store fish) for not using one.
Just my 2 cents.