Not a steal necessarily. They have to feed it, keep the tank they're in presentable, pay an electric bill, pay their staff, and pay rent and then assume the risk if something dies or kills its tank mates. Then there's the fact the aquarium they put those guys in could have housed a bunch of small fish that actually net more of a margin.
Most folks who own a LFS would like to pay the bills at home and probably have some mouths to feed as well.
Obviously, there weren't a lot of people looking for what you were willing to sell, or you would have sold the fish directly and pocketed the difference. Most aquarists don't have big tanks to house big fish in either.
