Was he inside the feed store (in which case you should let them know that the chicks they are selling might not be healthy, and perhaps they'll give you replacements) or was he just in the parking lot selling them out of the back of his truck (in which case its likely that they weren't raised or fed properly).
watch their behavior- if they are piling together and on top of each other, they may be too cold; if they are by themselves and as far from the light as they can get, they're too warm. You're doing the right thing with fresh food and water, make sure they are eating and drinking; you might have to dip their beaks in the water once or twice to teach them where it is.
How many chicks do you have and how old? How did they act before dying might give a clue as to what happened and how to fix it. What kind of feed are they getting.Just a quick run through on the heat. For the first week they need an area about 95 degrees by the heat source to warm up if they get cold. You can drop that requirement about 5 degrees per week till they are at ambient temperature. Also don't heat their whole area, they need a cooler area if they get overheated. Sorry to ask so many questions, just concerned for the little ones and want to help. Best of luck with the rest of your little ones.