The supplier is probably at fault, but it is the retailer that must take the responsibility. It is the retailer that decides to order, stock and sell particular items. If the Wholesaler's product is sub-par then the retailer is at a point where he/she must decide whether or not to keep dealing with the supplier, to cut current deals and find another, more responsible wholesaler. So while it may be the supplier's fault, and they will suffer in business from actions like this if it continues, it is the retailer that takes on the responsibility of selling, or not selling it to the public.
Another thing to think about is what the species were. You said these were rare fish that aren't carried in LFS stores often, perhaps there is a reason for that. If these were rare, sensitive species of fish then they may suffer from diseases in captivity that others do not due to stress that more common fish do not suffer from. Say if it was a shipment of Altum Angelfish compared to a common Tiger Oscar. The mortality/disease rate of the rare, sensitive fish may just be higher.
Just my thoughts...
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