For the mislabled fish comments - there is the legal end of it a company has to keep up with. If they get fish X shipped to them as fish Y, they have to sell it as fish Y to match the invoices and keep the paperwork in line. Otherwise it would appear as though they were creating fish and selling them for profit with no documentation. IRS doesn't like that at all.
As for the question of mortality, most stores shoot for under 30% mortality (death) when comparing what sold vs what they brought in. Good stores will get that number under 20%.
Even in the public aquarium world, 5-10% is the goal. There are factors outside of the end keepers control (age, conditions transported in etc...). You will never get to a 0% loss. Shipped fish going into over stocked retail tanks, then add the stress of constantly having nets chasing them around, constant introduction of new pathogens etc... Retail fish tanks are nothing like the nice stable tanks we keep at home. Imagine how upset you'd be if someone came into your house, chased your fish around five or six times a day with a net (stirring up the bottom, knocking decor over), then twice a week dropped in new fish from various sources. Kinda makes you think, huh?