From within an Inventory record click on the Transactions tab and then click on the green edit button 
In the Status field drop down box choose Sold, Reserved or Consigned (if your item is already sold you can reverse the sale by choosing Available). For Sales and Reservations set the price at which you will be selling; for Consignments set your percentage or actual charge to the Consignee if they sell the piece.
You can ‘transact’ with any of your existing Contacts or you can create a new Contact record during the process.
The video below is a detailed walk-through of a sales transaction.