Towny's economy is based on the Gold Standard. Gold is obtainable from Gold Ore, or Nether Gold Ore (drops 1 Gold Nugget).
Players make money by selling Gold Ingots to Treasurers. Treasurers are NPCs at Spawn and in Server Outposts, that allow players to sell Gold Ingots for $6 each, and buy them for $12 each. However, nation members can sell and buy Gold Ingots in Server Outposts that their nation controls (after a successful siege) for $9 each. You can check the Server Outposts that your nation controls by typing /controlstatus.
To transfer money from your balance to the balance of another player, use /pay (name of player). You can enable or disable other players being able to use this to pay you by typing /paytoggle (enable/disable). Also, you can enable or disable confirmation messages when using this to pay other players, by typing /payconfirmtoggle.
A playershop is a chest that buys and sells items automatically for a player, even when one is offline.
Click here for more information on playershops.