what's the difference of the two trading engines? Hi was reading through the wiki about exchanges, 

https://en.bitcoin.it/wiki/Trading_bitcoins

it looks there are two kinds of them, what's the difference?
The price in real-time trading exchanges is determined dynamically based on supply and demand. People sell and buy which influences/determines the price fluctuations.

The price in fixed rate trading exchanges is whatever the seller chooses to; e.g. selling 1btc for $450 each. That price is usually determined by consulting real-time exchanges and the seller will update the price accordingly.