Genuine boards are sold by the people on the distributor list (which are listed here: http://arduino.cc/en/Main/Buy
) for about $30 for an Uno.
I don't know if ANY of them sell on eBay, and certainly not for half the normal price.
So that means that nearly all of the boards you see on eBay are clones.
This divides into approximately three types:
1) Value-added clones. There are some vendors who offer Arduino compatibles with assorted added features, different shapes, or other changes. Seeeduino. Freetronics. Adafruit. Modern Device Corp. Sparkfun. (note that several of these vendors sell both Real Arduinos AND alternatives.) These are mostly in the true spirit of open source; they've taken the reference design and DONE things to it that make it better in some way (from someone's point of view.) Sometimes cheaper, too.
2) Minimal-effort clones. These are simple duplications of the Arduino reference design, sometimes exactly as published, sometimes with a new name or silkscreen. (note that the PUBLISHED Arduino design does not LOOK like the official Arduino; it doesn't have any of the fancy silkscreen, for instance.) They should be marketed as being "Arduino xxx compatible" rather than "real." These aren't so meritorious as the value-added clones, but at least they get the basics trademark law and the Arduino team's desires.
3) Deceptive clones. Marketed as true Arduinos, with a board (and perhaps even packaging) designed to look a much like a real Arduino as possible. Also known as "counterfeits." Sleazy scum; if you ask me. Anyone who goes to that much effort should have put it into doing something useful instead of counterfeiting. A minimal-effort clone has value if sold through channels where normal Arduinos aren't sold, or even if it's just at a lower price. A counterfeit is just ... wrong.
That sorta means that if you find a $15 Arduino on eBay, the closer it looks to a real arduino, the worse the vendor is behaving. An annoying paradox.