Be very skeptical about the health of the fish. If there is a sick fish in a tank the others may be sick as well. If you do find a fish in a tank with others that are ill treat the well fish as you would the sick ones which you did not purchase. Only a small percentage of a goldfish spawn will grow to be quality fish. Most of the goldfish seen in petshops fall into this group.
Goldfish breeders cull their fish and sell the results to the fish stores.
Some end up as feeder fish others end up as small fancy fish with problems that were obvious to the fish breeder but may not be to you. The breeder sells the fish as soon as it is clear that it will not develop into what he considers to be a good fish. The changes of buying a small fish at a pet store and have it grow to be a prize specimen is small. Your changes are much better with larger goldfish because what you see is what you get, the majority of the development is done. When buying bubble eyed goldfish avoid fish with bubbles that are too large or to small.