Beware, young Alice! That rabbit-hole you see is waaaay too deep!! Your seemingly innocent question is indeed very profound and stays at the intersection of history, philosophy and economics.
Whence do anything get its value? According to the Austrian school of economics, there's no such a thing as an "inherent value" of stuff: all value is subjective, we as economic participants "imbue" value onto the things according
to both rational and irrational beliefs. Should we be in a desert island, would you rather have 3 gallons of water and a wooden raft or 100 kilograms of pure gold?
Bitcoin itself had imbued value way before there were exchanges set for it. People would give each other coins as a sign of appreciation, a kind of "membership key" into an über-exclusive (and ultra-geek) club or, as in an (in)famous example, in exchange for some pizza.
As for buy/sell orders moving the price of a coin: yes, that does happen when the market for that coin doesn't have too much volume. For instance, with millions of dollars in bitcoins changing hands daily, a single buy/sell order for 1 million USD at any exchange won't move the price up/down more than 5-10%, and that "hiccup" will rapidly correct itself. Try that with, say, Litecoin and the price would go up about 33-50%, and most probably such movement would start a rally where people and automated trading bots enter an irrational frenzy trading, trying to ride the wave.
For a very dramatic and recent example you can check the Protoshares market at cryptsy - look up the graphs for 17-Feb to 18-Feb. And the equally impressive dive (plus its damping up and down the new equilibrium price) about 12 hours later. Unobtanium, Feathercoin and other minor coins have recently experienced similar (induced?) rallies.