Healthy hatchlings are very alert and very fast. So that's exactly what I would look for when buying one. I actually held off on handling my female until she's the age she is now, 5 months. One because they're very fast, and two because of how fragile they are at that age, they are more prone to drop their tails at that age. I just did the usual health checks (monthly weighing) and cage cleanings. Also they are very good eaters. Mine took a special interest in crickets. If possible, I'd definitely see how good of a eater he is. Examine the belly and vent of the gecko. A healthy gecko has pretty good sized, rounded belly, make sure you don't see any ribs (that look like he hasn't been eating). If he has caked or smeared feces around his vent, don't buy him, that's a possibility of a gastro-intestinal disease. Check his tail, there shouldn't be any kinks in his tail, if there is, that can be a sign of calcium deficiency. I would also take a look at his feces. It should be pretty solid, black and have some white on the end (urates) if it's smelly and runny, he most likely has parasites. That can be taken care of by a vet, but if it were me I would make them take care of it before I bought him.

One more thing, since you have other reptiles, when you buy the little guy make sure you quarantine him for at least 60 days. When feeding your other reptiles, feed him last. Never recycle food, such as crickets from one gecko to another. Also make sure when you handle him, wash your hands before touching or handling another reptile. If he has any disease, you don't want to spread it to the rest of your colony.