How can I distinguish between a clone and a real Arduino?
Trust your instinct and don't buy from eBay. There's no way to verify authenticity until you have the board in your hands, and even then it can be tricky to know for sure.  The official boards are packaged beautifully and have stickers and instructions... they're not impervious to counterfeiting, but it's a lot of work for not a lot of counterfeiting payoff.  If you order one from ebay and anything in the packaging doesn't look right, it's probably not an official board.

Amazon does sell authentic boards manufactured by Arduino, so pay close attention to who the seller on Amazon is.

There are many many clones and variants out there that are probably perfectly fine to use if you are confident in your arduino fu. So don't be afraid to buy a variant like the EMSL Diavolino or the Modern Device BBB.  However, if it's a sketchy 3rd party seller on ebay and it looks like a fake, it probably is. Go with an official reseller of official boards, or buy a well respected variant.