A Bitcoin is worth whatever someone's willing to exchange for it, be it US dollars or peanuts. The same is true of any other currency or item. To suggest that the exchange rate on MtGox or any other exchange sets the value of a Bitcoin is a very limited view.

As for the price on Ebay, if one were to purchase a Bitcoin on MtGox for $12.00. turn around and sell it on Ebay, it would need to be sold for around $14 in order to break even because of the fees.  $20 is not unreasonable considering the potential for charge-backs.