The way haggling works, you don't actually have to buy anything. Just keep clicking on haggle (make sure the skill is highlighted) until you either succeed or the shopkeeper refuses to sell you that item. Then exit the shop interface, click "buy/sell" again and the shopkeeper's disposition will be reset. Each time your skill will increase by a few points (more if you succeed, less if you don't). You can do this all day and increase your haggling skill to 100 in one shop without spending a single coin.