A smart store feeds pre-killed rodents only. Again, this is very general. Each species is different in care and food requirements. Shells should be free of pitting, or peeling shell that has exposed bone. Some aquatics will shed the top layer of their scutes, the individual pieces of shell, but that is one thin, clear layer that has new shell underneath.
Sunken eyes are bad, as is a turtle that does nothing but bask.
Softshells are a special problem. They need VERY clean water, and a smooth bottom, either fine sand, or no substrate at all. They are prone to abrasions and fungal infections - small round ulcers on the shell and legs. They should not be kept with other turtles, as this can lead to scratches and infection. While swollen eyes can occur in any turtles, they seem very prevalent in box turtles.