Reference for Bava Metzia 4:4
אֶחָד הַלּוֹקֵחַ וְאֶחָד הַמּוֹכֵר, יֵשׁ לָהֶן אוֹנָאָה. כְּשֵׁם שֶׁאוֹנָאָה לְהֶדְיוֹט, כָּךְ אוֹנָאָה לְתַגָּר. רַבִּי יְהוּדָה אוֹמֵר, אֵין אוֹנָאָה לְתַגָּר. מִי שֶׁהֻטַּל עָלָיו, יָדוֹ עַל הָעֶלְיוֹנָה, רָצָה, אוֹמֵר תֶּן לִי מְעוֹתַי, אוֹ תֶן לִי מַה שֶּׁאוֹנִיתַנִי:
Both the buyer and the seller can claim ona'ah. Just as a non-merchant can claim ona'ah, so can a merchant. R. Yehudah says: A merchant cannot claim ona'ah. [For he is expert (in wares) and it may be presumed that he waived any ona'ah. As to his having sold it for that price (in the first place), it is because (the opportunity for) another sale arose, and now he wishes to back out. The halachah is not in accordance with R. Yehudah.] Whoever claims ona'ah has the upper hand. If he [the seller] wishes, he can tell him: "Give me my money," or "Give me the amount of he ona'ah."