Answer: When people buy something, they value it more than whatever it costs them, when people sell something, they value it less than the payment they receive.
Explanation:
For a commercial exchange to happen, the buyer must consider the item being bought as more valuable than the amount of money they have to spend to buy it. Likewise, the seller must consider that item as less valuable for themselves than the money they are going to get for the exchange. The seller even establishes a price that it´s above the actual cost of that item, to get a profit.
