most people with green icons above their heads will give you the option to buy or sell when you talk to them. just choose sell, and choose 5 items that you don't want to keep and you will get the achievement. note this has to be 5 different items, not 5 items in one stack. you can click on your face to bring up your inventory, and then hold on an item to get the option to split it. maybe someone can confirm that you can sell 5 of the same item after splitting it.