Fence

This achievement is really easy to get. As you progress through the game you will find loads of weapons, most of which will be worse than your equipped ones. A simple way to get this achievement is to pick up and sell any weapon that you find and don't want to keep rather than just leaving it. This way you can easily get this achievement by the time you get to New Haven and will also give you lots of money helping with the Pandora-dog Millionair achievement.