The General Store is the primary shopping location, and there is at least one in each city. This is where you'll find a Shopkeeper.
To buy items at the General Store, right click the Shopkeeper. He will display a list of items in stock, and the player simply clicks on the item he wants.
You may also use the command
to access the store, though this is likely to be removed.
To sell an item to the General Store, hold it in your hand and punch the Shopkeeper. The money you receive should be equal to the number written in the lore. Currently this mechanic is a WIP and therefore a lot of items cannot be sold.