童叟无欺 Cheating neither child nor old man. Fair and honest trade.
货真价实 Genuine goods at true prices.
和气生财 Friendliness brings wealth. A dictionary calls it a common saying of traders: a seller who is gentle and does not offend customers will get rich. In Lu Xun’s story Divorce, a character says: “A person should always be friendlier. ‘Friendliness brings wealth,’ isn’t that right?”
货有高低三等价,客无远近一般看 Goods come at three prices, high and low. Guests near or far are treated alike. In Journey to the West, Sun Wukong quotes it as a common saying to an innkeeper who has just named three classes of service.