First off *hugs* because losing chicks is always hard.
Second, when you buy chicks from someone and they all die, chances are that you shouldn't buy from them again
Definitely don't go back and buy from him again this time, 2 strikes is a seriously bad sign.
If you think it might be environmental, then there's a lot of questions to answer first before anyone can possibly tell you what might have happened.
How old were the chicks?
What were you feeding them? Have you checked your feed for mold or other hazardous agents?
What were you watering them in and the sort of water you were giving them?
What sort of bedding do they have?
What temp is your brooder at? Is it near drafts? Direct sunlight?
How were the chicks behaving before death? Anything the slightest bit out of the ordinary could indicate what had happened.
Did you check for mites and worms?