I generally use shapeshift.io I did not realise that they do not affect exchange prices as much, so I'm pretty pleased to learn that it's a good thing. I have a wirex card so it's very convenient, they have already got the link to generate a shapeshift trade on the e-coin.io site.
So now I'm also going to be advocating the use of shapeshift and blocktrades. Thanks for the info.
All the simple instant exchanges mostly provide operations via Poloniex or Bittrex. Our service http://Changelly.com also do the same. And of course, each deal moves the market price. You can have a look, how it works, and how do we calculate rates here: https://steemit.com/money/@changelly/how-changelly-suggests-the-best-exchange-rates
They do, but they hold a buffer of funds and adjust them. In fact, idk about blocktrades, but shapeshift probably need more people and a bigger pool. They are OTC exchangers, like the change bureaux that you see in especially capital cities and anywhere near large airports. Their money changings do not affect the exchange rate that much, because their business is all about instant exchange. So they hold a big pool of all the currencies.