This is not the fault of the administrators of Buy Sell or Trade, who simply created a place for people to trade goods freely. This is the fault of the rabid gun culture in the United States, that claims firearms are just like any other product the trade of which should not be infringed by "big government," as well as the fault of a well-established conservative government bloc eager to please their National Rifle Association cash cows.
Guns are not simply products. For while one individual may be the owner of a gun, it is an instrument that affects the public. These can take a human life in a far more simple manner than a knife, or a car, or a gas stove. We must stop kidding ourselves and realize that the only way to keep guns out of the hands of bad people is by making strict laws on who can carry a gun and a system of stringent checks that must be completed in order to contain an object capable of such terrible atrocities. If that means they cannot be sold on the internet, where making such checks are more difficult, then so be it.