Why, resale value of coins. The higher the sold price of a coin is, the more it drives up the market. The lower the historical sale price of a coin is, the more it drives down the market. So, let's say I have a coin in my collection and I see another one selling for less money at auction. I can let it go and someone can pick it up at prices I can never pick up coins at, or I can bid at a price I am willing to pay for the coin and add it to my collection. If the other bidder wants the coin more than I am willing to pay for a duplicate, they can have it.