What does the Torah tell us about honesty in business dealings? Are there any guiding principles for Jews in honest business dealings?

The Jerusalem Talmud (Bava Metzia 2:5) states it powerfully (what follows is a free translation):

Rabbi Shimon ben Shetach dealt in linen. His students said to him: “Rebbi, desist from this trade. We will buy you a donkey [to make an easier living as a donkey driver] and you will not have to toil so much.” They went and purchased a donkey from a bandit. The students subsequently found a precious stone dangling from it. They went back to Rabbi Shimon ben Shetach and said to him: “From now on you need not exert yourself.” He asked: “How so�” The students responded: “We purchased a donkey for you from a bandit and a precious stone was dangling from it.” Rabbi Shimon ben Shetach asked: ‘Did the donkey’s seller know that the stone was there?” They answered: “No.” He then said to them: “Go return it.” The students remonstrated with Rabbi Shimon ben Shetach: “Although theft from an idolater is prohibited, is one not permitted to keep an object that an idolater has lost?” He responded: “What do you think, that Shimon ben Shetach is a barbarian? More than all the wealth of the world, Shimon ben Shetach desires to hear [the non-Jew say]: “Berich Eloko d’Yehudo’ei” (“Blessed is the God of the Jews”).

Our paramount value must be Kiddush Shem Shomayim (sanctification of God’s name).