When buying small discus - choose ones with small eye to body ratio. There fins should be up and they should be begging for food. Look at the overall health of the fish. The fins should be "up" they should be active. Look for obvious deformities on the gills, mouth etc. Make sure they are breathing through both sides of there gills and watch them for 10 min or so to see if they scratch, flick or show signs of irritation.
The tanks should be clean, also look for other fish that are sick. One sick fish is enough for me to say no.
People buy extra fish when buying small ones because you really don't know what you are getting until the fish are about 4 inches. Up until then, they are still developing bones, shape and colours and could turn into something unexpected. For me its not an issue, so long as my fish are health and pretty I'm good, i don't need show quality fish to enjoy them.
Like any Cichlid, be wary of small fish with bright colours. They generally don't show adult patterns until they are 4 inches + and from what i understand it brightens and intensifies until about 2 years of age.
Hope this helps, and I really hope that you know what is required to add size to small discus.
EDIT - dont buy dark ones, they are stressed.