When buying a used or surplus transformer one should consider the following:

• Know that what you are buying is the correct unit for the application.  If you're not sure, just ask!  A reputable dealer would be happy to answer any questions and steer you in the right direction. 
• A transformer is an electrical device that is used to change voltage and amperage.  It does not generate power. 
• In addition to pictures of the outside, make sure you ask to see pictures of the core and the coil on the inside of the unit.  The core and the coil will rust faster than the sheet metal on the outside of the transformer.  Some rust on the outside of the unit will not inhibit its functionality.  Asking to see pictures of the inside of the unit also allow you to determine if the coils are aluminum or copper.  We typically only post one or two photographs of our transformers to keep listing costs down but we are happy to provide as many pictures as necessary to make the potential buyer feel comfortable about their purchase.
• Finally, remember that these items will almost always be shipped by freight carrier so keep that in mind when purchasing.  Transformers are heavy and must be secured and packed correctly.