Silver Coin

Silver Coins can be crafted from 100 Copper Coins (which can be obtained by killing monsters or smashing Pots). You can also use 100 Silver Coins to make a Gold Coin.

A Merchant requires a total of 50 Silver Coins (or more) and vacant House to move appear.

Once the merchant has moved in, you can buy things from him with Coins

You don't need a Merchant specifically in order to buy things; you can also buy things from other NPCs, although by the time you have other NPCs that you can buy things from, you will probably already have the Merchant because he is one of the easiest to get to move in.