Couple of things I do when purchasing a fish no matter where it comes from.1 Have them feed it, watch how it reacts to the person feeding it and how it eats especially if its by itself. If its shy but eats it may not get food if you have more aggressive feeders. If it doesnt eat well ..... dont buy it good chance it wont eat for you either. It shouldnt show fear it should be happy to be getting food.2 if its doesnt have a date on the tank of when it came in ask how long they have had it. If they dont know or its been less then a week I stay away from that purchase. 3 if other tanks are tied into the system how do the other fish act, look, great breeding place for parasites. Find out if they run copper or any meds in the holding system. And the one thing I have not done and should also do is set up a QT.
Sorry to hear about your bad luck. I have had good success following those steps. I did bring ich home with a fish once and the lfs I got it from QTs everything for at least a week before it hits the floor so a QT is always a good idea.