Chullin 94a
חולין צ״ד א — ד"ה תָּנוּ רַבָּנַן
Chullin 94a:14
The Gemara discusses geneivat da'at, deception or misleading another person's mind, including misleading a buyer. This is a foundational source for honest sales behavior.
תָּנוּ רַבָּנַן: לֹא יִמְכּוֹר אָדָם לַחֲבֵירוֹ סַנְדָּל שֶׁל מֵתָה בִּכְלַל שֶׁל חַיָּה שְׁחוּטָה, מִפְּנֵי שְׁנֵי דְבָרִים: אֶחָד – מִפְּנֵי שֶׁמַּתְעֵהוּ, וְאֶחָד – מִפְּנֵי הַסַּכָּנָה. וּמִפְּנֵי שְׁנֵי דְבָרִים אָמְרוּ אֵין מוֹכְרִין נְבֵילוֹת וּטְרֵפוֹת לְגוֹי: אֶחָד – מִפְּנֵי שֶׁמַּתְעֵהוּ, וְאֶחָד – שֶׁמָּא יַחְזוֹר וְיִמְכְּרֶנָּה לְיִשְׂרָאֵל אַחֵר.
The Sages taught in a baraita: A person may not sell to another a sandal made from the hide of an animal that died of natural causes as though it were a sandal made from the hide of a healthy animal that was slaughtered. This is prohibited due to two factors: One, because he misleads the customer into thinking that the leather is of higher quality than it really is; and another, because of the danger involved, as it is possible that the animal died from a snakebite and the poison seeped into the part of the animal’s hide from which the sandal is made. And it was due to two factors that the Sages said that one may not sell meat from unslaughtered animal carcasses or tereifot to a gentile: One, because it misleads him, as he thinks that it is kosher meat, which is more desirable; and another factor is the concern lest the gentile then sell the meat to another Jew, who will think it is kosher since it originally was sold by a Jew.