Since the era of Mt. Gox lots of things have changed in the crypto world. Bitcoin is not the only kid on the block, exchanges have open and closed, market is not just the spot market we used to know and so on. If you are reading this, you know there are more ways to get some crypto, other than buying or mining.

However there is one thing that hasn't changed since the era of Mt. Gox; the order book. Specifically, not the order book itself, but the way exchanges execute orders submitted to the order book.

If you keep a securities account, or you have traded on FX rates in fiat currency - even by using a demo account, you know there is an automatic system matching the best purchase order with the best sale order, for a transaction to be completed. For many centralised exchanges and all decentralised exchanges, this is not what happens. One - or many, who wish to fix the price for a specific pair - may select an individual order for execution that is way below or way higher than the best price. Some decetntralised exchanges are going one step beyond that: not only they do not have a matching order system in place, but they also use terms that confuse the user, making him/her believe that the number displayed is the gas fee.

The best way, however, to get a glimpse of the other, darker, side of the crypto-exchanges, is how they make their money. Because all crypto-exchanges are making money, even those that claim they don't charge any fees. The secret is in the order book. The order book is a table that users rarely have access to. Even if they have, the rates shown are never the rates submitted by those who place the orders. Larger exchanges - like Coinbase - do list the rate they use as a footnote. Smaller exchanges - especially the decentralised ones - present (or even advertise) offered rates as the actual rates submitted by buyers or sellers. They are not. The contract they use imposes a hidden increase or decrease at the offered or asked rate. That's usually 1% of the offered or asked rate. So, if a user is offering a WIDGT for ETH at a rate 1:1, the rate is presented to the ETH seller as 0.99:1. Or 0.98:1. Or even 0.95:1. Since decentralised exchanges do not have to follow market rates (because it's the users who are submitting the rate they wish to trade), the displayed rate to one side or the other (or even both), is whatever rate the buyer or seller is submitting, plus the hidden exchange markup.

So, beware of the exchanges claiming they charge no fees. Usually, their rate markups are higher than the highest commission you could ever pay to an exchange with the markup displayed publicly.