I would have a necropsy done on the dead bird. I'm sorry for your loss, however if you intend to sell the chicks and have had a recent death, you need to know what the bird died of in case it was something contagious that can be spread to one of your buyer's flocks. Also if it was bacterial, you can have a culture done on those chicks to make sure they don't have it or get them treated before symptoms break. Also, because it seems like you are bringing birds in from a breeder, that ups the disease risk for your flock. Personally I'd be having a little anxiety attack if I frequently brought in new birds from other people and one of my older birds died. Piece of mind is golden for me and I always do the necropsy unless its just a ridiculously old bird with a previously diagnosed problem. Sudden deaths always get necropsied at my house. Thankfully, its been several years since I had one of those. Birds don't just die for no reason. It could be a complication related to diet, something in the air, fungal, viral or bacterial infection, cancer, blood clot etc.
You might want to try posting on the breeder forum on this website to reach sun conure breeders for the weights. I don't do conures myself.