Ben: Don't rip people off. Because people should get what they pay for. Don't treat your friends differently than other because you know them better and you are treating them nicer. Also don't treat others worse.
(יב) קָשֶׁה עָנְשָׁן שֶׁל מִדּוֹת יֶתֶר מֵעָנְשָׁן שֶׁל עֲרָיוֹת שֶׁזֶּה בֵּינוֹ לְבֵין הַמָּקוֹם וְזֶה בֵּינוֹ לְבֵין חֲבֵרוֹ. וְכָל הַכֹּפֶר בְּמִצְוַת מִדּוֹת כְּכוֹפֵר בִּיצִיאַת מִצְרַיִם שֶׁהִיא תְּחִלַּת הַצִּוּוּי. וְכָל הַמְקַבֵּל עָלָיו מִצְוַת מִדּוֹת הֲרֵי זֶה מוֹדֶה בִּיצִיאַת מִצְרַיִם שֶׁהִיא גָּרְמָה לְכָל הַצִּוּוּיִין:
(12) The punishment for the person who uses false measures is greater than the punishment immoral behavior, for this is a sin between a person and his colleague, and this is a sin between a person and God.
Whoever denies the mitzvah of just measures is considered as if he denied the exodus from Egypt, which is the first of God's commandments. Conversely, one who accepts the mitzvah of just measures is considered as if he acknowledges the exodus from Egypt, which brought about all of God's commandments.
From Torah Musings: Equal Weights and Measures, Rabbi Gidon Rothstein: Terming weights and measures a moment of judgment sheds light on both sides of the equation. It tells us judging is about more than formal cases, it is about the decisions that foster society’s smooth running. Adjudicating disputes, catching and punishing criminals, these are all vital to society. So is being able to trust the market.
From EtTzion.org, Exact Weights and Faith in God, by Rav Binyamin Zimmerman:
Why are these mitzvot of such fundamental importance? Obviously one must not use illegitimate weights, but is this not simply another aspect of not depriving another person of his property? Any such act, we would think, should qualify as robbery or theft, both of which are prohibited earlier in Chapter 19 of Vayikra. What, then, makes inexact weights and measures unique, and what special connection do they have to righteousness and justice? Additionally, why should simply not possessing such weights and measures be reason to deserve longevity, and why should simply possessing them render one an abomination?
Further, what follows these mitzvot is of note. In Vayikra, they are followed by a description of Gods taking us out of Egypt, while the verses in Devarim are followed by the mitzva to remember what Amalek did to the Jewish people. Why is this?
Chloe: Giving false measurements is being dishonest and lying to people about what they are getting. This is in the Torah because it is about being honest and trustworthy and fair.