Salesman of the month

Salesman of the month is one of the extraordinary skills. You're an expert in selling your wares. Your prices are so fair, it's incredible. Not the normal definition of fair, in which both sides trade something of equal value, of course, but the definition of a successful trader: your customer makes you rich. The selling price of your wares is 10% above that which you'd get following the normal trading rules. (That means you first apply the normal rules, then add ten percent).