This is a secret I found out on how to make money! but it does require at least 10,000 gold for the minimum... the amount varies.
once you coughed up enough gold, go to a merchant, anyone at all (not ones that sell weapons or cloths, the ones in taverns or on the road, i prefer the ones on the road, up to you though) and trade with them, but don't sell anything, buy every single item they have. this will raise the prices on everything you sell him... you must sell him everything you got. But not by one by one, use the 'Sell all' option, this will make you get more money for what ever you sell! and once you buy your stuff back, you get more money!