If an armed citizenry isn't what the Founding Fathers had in mind, then why didn't they ban the selling and possesion of firearms?

As far as military verses civilian arms goes, up until the 1960's, civilians had the edge in personal fire power. The standard weapon of the early U.S. army was a smooth bore musket, while most civilians owned rifled muskets. During the mid 1800's the army had rifled muskets and single shot breach loaders while civilians owned lever action repeaters. In the early 1900's the military used 5 shot bolt actions and revolvers, while civilians had 15 shot lever actions, semiautomatic rifles and pistols, and could even own Tommy guns. Not until the 1960's did the military gain the upper hand in personal arms.