The buyer and the seller are obligated under the laws of ona'ah. They cheat each other. Let’s say the seller overcharges or the buyer underpays. He negotiates a price that is super low by an amount equal to one-sixth of the item’s market value. Then the other side could demand a refund.
Just as the laws of ona'ah apply to a regular individual, they also apply to a merchant. Even though a person would think that a merchant, somebody who’s in business, understands the market. If he’s buying something, he knows what it’s worth. Therefore, we could assume that he forgave the amount of ona'ah. The Mishnah comes to tell us, no: you’re not allowed to rip off a merchant either. Rabbi Yehuda omer, but Rabbi Yehuda says, “No, the laws of ona'ah do not apply to a merchant.” A merchant knows the market value. We could assume that if he bought it for a different price, he forgave the amount of ona'ah. Maybe he needed quick cash in order to buy more merchandise. We can’t really understand why he did it, but he was aware of the situation he got into, and he paid that amount for a reason.
The Mishnah comes to tell us, no: the person who was taken advantage of—either the purchaser who paid too much for an item, or the seller who was underpaid for an item—always has the upper hand in how to settle it. He could say, “Give me my money back. Give me a full refund, and I’m going to give you the item back.” Or he could say, “Give me back the item you cheated me with, and I’m going to keep the item.” In other words, give me the funds back. If you overcharged me by four ma'ah (coin), give me my four ma'ah back, and we’ll be fair. I would have paid market value now, and I can keep the item. Then we’re all fair. He has that choice: a full refund in exchange for the item, or just getting back the funds he was overcharged.