i think exchangers buying it when the price just seems so profitable,and i think how exchangers could get profit is like when people doing trading,buying at the low price and selling it in the higher price,thing that make it different,exchangers can't sell the bitcoin immediately as traders does
I think exchange does not have to trade to get the bitcoin. It can just get the bitcoin from the trading fees.
an exchange that trades would be not very smart. because nobody can say where the price will go and so there would be the possibility of losing money.
to start an exchange you need some money anyway. a part of this you will invest in a couple of bitcoins to start with, but here is how exchanges usually work.
the bring a seller and a buyer together and just exchange the goods and take a cut. so they can never make a bad deal and the profits just have to be bigger then your cost.