Why people would sell you something for bitcoin? Because they believe that later someone will buy that bitcoin from them.
And since a bitcoin is not a directly serviceable good, that is the *only* reason to buy bitcoins. It's commonly known as the "greater fool theory":
http://en.wikipedia.org/wiki/Greater_fool_theoryIsn't it nice to know that the entire "bitcoin economy" is based on everyone expecting to find a greater fool to buy their bitcoins?