The player market is a feature we use so that players can sell and buy from each other. You can build the player market anywhere within your town and invite others to shop at it.
You can buy from another players shop by left clicking on the sign with the stock. This will give you information in the chat. You enter how many items you want to buy by typing the number in the chat or typing in all to buy everything.
To set up your shop you will need a chest and items to sell. Once you place your chest in your purchased plot, you will need to left click it with the items you want to sell. It will ask you in chat how much you would like to sell your items for and you just need to type in a number.
After you do all this your shop will look like this;