Am I always buy/selling to a peer?
Or does the exchange sell and buy directly to users also?
Does an exchange adjust its prices in consensus with other exchanges?
Or is it all independent marketing?
The two exchanges Bitfinex and Bitstamp used to have a deal that helped keep the price roughly the same on both exchanges. However, they stopped the deal now as far as I know. There are people who run things called arbitrage bots which look for different prices on exchanges and make profit from buying off a cheaper exchange to sell on a more expensive exchange. They tend to keep the prices roughly the same across exchanges.