While Crafty's is the largest and most popular shop, players are able to setup their own shops. In fact, you are seriously encouraged to create competition.
Find a sign/chest with the product you have to sell or wish to buy. Left-click the sign to sell the product to the shop, and right-click to buy.
Creating a Shop
To create a store, simply place a chest with a sign above it (you must be in a towny plot that allows shops). The sign must follow this format:
Line 1: Your username
Line 2: Quantity to sell/buy at once (ex: 64 for common items, 1 for rare or single-use items)
Line 3: Selling Price : Buying Price (The price you sell that quantity for, colon, the price you pay)
Line 4: The name or ID of the item. (ex: 1 for stone, or just “stone”)
That’s it! People can buy/sell items to you without you having to be present, even when you’re offline!