Can someone please help me understand what's going on in this order book? This is a screenshot of the XRP/USDT order book from Poloniex today. I have often noticed that there is a sell order and a buy order of almost the same size (usually quite large) and I think it is the same person doing it (either manually or using a bot)
The prices are different.
If you have a buy order of 100 units at price 1, and a sell order of 100 units at price 1.10, then your profit is the difference less trading fees.
This is how market makers make their profits.