The blackcksmith is one of only two NPCs that appear in the Guild that buy or sell, the other one being the Food Merchant. He sells weapons, tools (i.e. pickaxes, axes and shovels, but not hoes) and armor. If you press 'sell item' it will open up a GUI with your inventory and the amount of gold coins you have, plus an additional slot to insert whatever you want to sell there. It doesn't tell you how much it sells for beforehand, so be careful not to put the whole stack if you don't know how much it sells for, just insert one and consider if it's worth it or not, if not keeping it. Before version 1.0.5, red flowers sold for an enormously substantial price. But in 1.0.5, flowers are nearly worthless. After you start your kingdom, a blacksmith can also be found on the lower level of the item shop.