The starting point is Shabbat 31a, where Rava teaches that the very first question a person is asked in judgment after death is whether he conducted his business dealings with faithfulness, making integrity in commerce the foundational test of a life well lived.
This is rooted in the pasuk that Chavakuk 2:4 states, that the righteous person lives by his faithfulness, which Makkot 24a identifies as the single principle upon which Chavakuk established the entire body of 613 mitzvot.
Later authorities describe faithfulness as a defining trait of character: Mishneh Torah, Human Dispositions 5:13 describes the Torah scholar's business dealings as conducted in truth and faithfulness, meticulous with himself while generous toward others, so that his word stands firm, and Kad HaKemach, Faith 1:1 extends this to demand that one's yes and no be true and that one stand by his word in all business dealings, comparing a doubled affirmation or denial to an oath.
Mesillat Yesharim 11:15 states that the Holy One desires nothing but faithfulness, citing several verses that God's providence rests upon those who keep faith, while Duties of the Heart, Fourth Treatise on Trust 1:1 explains that trust itself depends on confidence that the one trusted will keep his word and do what he pledged.
On the practical plane, Mishneh Torah, Sales 14:1 rules that one who buys and sells faithfully, openly stating his profit margin, is exempt from the laws of ona'ah even on a very large markup, showing that transparent dealing removes what would otherwise be exploitative.
The Shulchan Arukh codifies the concrete demands of faithful dealing: Shulchan Arukh, Choshen Mishpat 231:1 forbids any deception in weighing or measuring and treats it as an extremely severe transgression, while Shulchan Arukh, Choshen Mishpat 228:6 forbids deceiving people in buying and selling or misleading their impressions, including concealing a blemish in merchandise or creating a false impression of favor.
The Gemara in Bava Metzia 36b and Bava Metzia 94b builds the laws of the four types of bailees around the trust the owner places in the bailee, ruling for example that a bailee who transfers a deposit to another custodian remains liable because the owner trusted specifically him.
Mishneh Torah, Creditor and Debtor 1:4 extends the theme of faithful financial dealing to the collection of debts, ruling that a court must collect a creditor's due in full regardless of the debtor's hardship, reflecting the seriousness with which financial obligations are treated.