Pewter is an alloy of Tin, Copper and (historically) Lead.  Modern pewter is made with Antimony or Bismuth in the place of Lead, in order to make it safe to use with food.  There are many modern manufacturers of pewter, from well-known corporations such as Kirk-Stieff and International, to smaller producers of handcrafted items such as Gibson, Woodbury and Danforth.  All of these companies produce true pewter, and it is often marked on the piece as being 'genuine' pewter. 

However, there are other companies which produce items supposed to be pewter by many eBay sellers, which aren't.  These items are made of differing alloys, most commonly an alloy of zinc or aluminum.  These 'mystery' alloys are called by a number of trade names, such as Armetale, Statesmetal and Pewterex. 

Including these names as negatives (i.e.; "-armetale, -pewterex") in your eBay searches will exclude most of these items from your results list.  I'm a bit of a purist, so I took the time to write this in order to assist other beginning pewter collectors.  Good Luck!