Used furnaces in limited quantities, not vetted by a professional, can be found for sale on eBay, who also has a selection of parts and wiring for furnaces.. More industrial used furnaces can be purchased at Aaron Equipment Company.
The market is sparse for used furnaces because of their general limited lifespan and their expensive installation procedures. Professional installation for a new furnace can reach several thousands of dollars if ductwork must be made for the unit. Any additional repairs that must be made for the unit over time are costly. Furthermore, the older the furnace, the less energy efficient it is and the more costly it is to heat the home.