The thing to worry about is bacterial infection. Put it on hold and check it every couple of days, the LFS should hold for up to 2 weeks if they want your business. Watch for dusting of the scalles, clouding of the eyes and make sure it eats. A feeder fish will do fine for demo purposes, but eventually you want it to go on frozen, not live food.
Again, the warning signs are: a white dusting on the body, cloudy eyes, loss of appetite and drawback from the rest of the tines. If just one of these symptoms is present-don't buy him. Also, don't try a Florence Nightingale and try to nurse him back to health, just move on. (I know from experience).