They sold you a gecko that had just hatched the day before?!? That is NOT a sign of a good store no matter how you look at it. The stress of that alone (car ride, new home, etc.) may have been enough to kill it regardless of temps or anything else. Generally speaking cresteds should be an absolute minimum of 4 weeks old before they are sold and even then, it should be clear that they are well established and good eaters (otherwise they should be more like 6-8 weeks minimum).

With the entamoeba, sometimes it takes weekes or months to kill a crested. It all depends on the individual animal, their stress level and other factors. Not eating or not eating well is a sign that they have it. A crested will never not eat or starve itself on its own unless there is something wrong with it and any significant weight loss should be taken seriously. Other animals in this store may appear healthy but be carring the entamoeba. It just happens that it is extremely dangerous to cresteds as it is something that their systems can't deal with the way other reptiles seem to be able to.

How old is the replacement gecko you got? It is not necessarily recommended to give hatchlings the medicated food just for precautionary measures, however in this case it seems to be necessary. You may need to feed it by hand to make sure it is eating some of it, but at the same time be careful not to stress it out if this is another fresh hatchling.