Trading in Fable is a simple affair once you understand the mechanics. For a given item, regardless of your Guile level, you should aim to purchase items at no more than 60% of their base value and sell them at no less than that level. Armour and weapons are rarely a good item to trade as there are never enough of them in stock to be profitable. Guile will improve the cost and get you more profit, however, it is only about 3.5% per level. This will help most on higher priced items, but be near 0 on low cost items. It is always good to improve Guile, however, for trading purposes it is better done mid-game as early on you won't have much gold to buy with and not a lot of locations to trade between.