My son is learning Kiddushin and is familar with the rule of "ain ona'ah l'karka'os", there is no issur ona'ah by a sale of land. I asked him on Shabbos how this makes sense when the issur ona'ah appears in the Torah davka in the context of selling land - the Torah tells us that the buyer and seller must calculate the number of years until shemita to determine the fair price of land and avoid ona'ah.
The Ramban on that pasuk offers a few approaches to resolve this question. When buying and selling metaltilin, if the discreprency between what was charged/paid is too egregious, the buyer or seller has a right to rescind the sale. "Ain ona'ah l'karka'os" may simply mean that the buyer and seller may not rescind the sale, but the issur of ona'ah still applies.