Reading Order Book Liquidity Like a Professional

The last traded price you see on an exchange reflects a single completed transaction -- it tells you nothing directly about how much you could buy or sell right now without moving the price. That information lives in the order book: the full list of resting buy and sell orders at each price level, and how large each one is.

A market with 'thin' liquidity has relatively little size resting near the current price -- a modestly sized order can 'walk the book', consuming several price levels and resulting in a materially worse average execution price than the last traded price suggested. This effect, called slippage, tends to be larger for lower-capitalization assets and during periods of unusual volatility, even on otherwise liquid assets.

A simple habit that professional participants use routinely: before sizing a trade, glance at the visible depth near the current price, not just the last price. If your intended size is large relative to that depth, expect meaningfully worse execution than the quoted price, and consider whether a smaller size or a different order type is more appropriate.

This is also why systematic execution engines typically cap order size relative to expected liquidity and, where relevant, compare the expected fill against a reference price rather than assuming perfect execution -- a detail that's invisible when a strategy is only being evaluated on a theoretical entry price.