This isn't true. "Each exchange charges a small fee for every trade" Not all exchanges charge fees for posting liquitity. Smaller ECNs will actually pay traders

This isn't true. "Each exchange charges a small fee for every trade" Not all exchanges charge fees for posting liquitity. Smaller ECNs will actually pay traders to provide liquitity. If you hit or lift someones bid or offer you will get charged a fee but if you post an order that does not cross against the current market you can get paided for. these rebates are very importatn for HFT desks.