Order Book Depth: Reading Both Sides and What Price Hides
A price tells you what one person on each side will trade at. Depth tells you how many of them there are. We cover what an order book is made of, why your own listing moves the price, and how to know in advance how many copies the market absorbs without slipping.
A price is one line; a market is two columns
When someone says "the item costs 30 dollars", they almost always mean the lowest sell listing. That is half the picture, and the smaller half.
Every live market has two sides. Asks are seller listings sorted upward, cheapest first. Bids are buyer orders sorted downward, most generous first. Between the best bid and the best ask lies the spread, and inside that gap no trades happen at all.
Depth is about how many sit at each level, not about which level is best. Two items both priced at 30 dollars can be built completely differently: one has forty more listings behind the lowest, the other has its second listing at 38.
The practical consequence worth reading on for: price answers "at what", depth answers "how many". For a single purchase the first is enough. For any repeatable operation you need the second, or you are calculating profit off a number that disappears with the first trade.
What the lowest price hides
It may exist in a single copy
The most common case. The lowest listing at 30 dollars exists once, the next is 34, the third 35. You see "price 30", you buy, and the market instantly becomes a market at 34. Nothing moved: a position simply ran out.
For a single buyer that is no problem. For somebody who planned to take five it is the whole problem: the average purchase price turns out closer to 35 than to 30, and the arithmetic of the deal changes beyond recognition.
It may be stale
A listing sits at the price that was current when it was posted. If an item clears one trade a week, the lowest ask can show a number for weeks that nobody today will buy or sell at. Depth hints at this too: a live item has dense, frequently refreshed levels, while a dead one has gaps yawning between them.
There may be nobody on the other side
The most underrated situation. Asks exist, bids do not. Which means selling quickly is impossible at any sensible price: no buyer exists at this moment, only a queue of sellers. The shop window looks perfectly normal — a price is shown, listings are in place.
In our data 91.1% of the rows on any marketplace are offers rather than completed trades. A one-sided market looks exactly like a two-sided one on the shop front: only the order book shows the difference.
How to read depth in practice
You do not need to be a market analyst. Four questions suffice, and an order book answers them in half a minute.
How many listings sit within 2% of the lowest price. That is your real headroom to buy without moving the price. Where the second level is. If the gap between the first and second listing exceeds your expected margin, the deal exists for exactly one copy. Whether bids exist and how far they sit from the ask. A wide spread with an empty buy side means your exit will be slow. How depth compares with daily volume. If your position is bigger than daily volume, you are no longer a participant in the price but the cause of its movement.
The last point matters most. It converts the vague "the item is liquid" into a number: compare the size of your intended position with the number of trades per day. A position larger than daily volume is sold not at the market price but at the price you create while selling it.
★ Karambit | Doppler
★ Butterfly Knife | Fade
★ Sport Gloves | Pandora's Box
Glock-18 | Fade
Desert Eagle | BlazeWhy your own listing moves the price
This is not obvious until it happens. By listing an item you become part of the order book — and if your listing is the cheapest, you have just set the new market price.
Then comes the race to the bottom familiar to anyone who has sold something common. You undercut your neighbour by a cent, the neighbour relists below you, you go lower again, and an hour later the bottom price has slid several percent while somebody else made the sale. On a thin item that race eats more than the marketplace fee.
Hence a working rule: on a thin market you set the price, not the market. Look not at "what it costs now" but at "what it will cost once I join". The difference between those two numbers is depth.
Compare both sides across marketplaces at once:
Depth and arbitrage
For a cross-marketplace deal, depth decides more than the size of the spread.
A 30% spread on an item with one listing on each side is a one-off trade for a single copy, and afterwards the deal is gone. A 6% spread on an item with a dense book on both sides is repeatable and scalable for as long as the gap holds.
| What you see | 30% spread, depth 1 | 6% spread, depth 40 |
|---|---|---|
| Profit per trade | larger | smaller |
| Repeatability | none | high |
| Risk of getting stuck | high | low |
| What happens after the trade | the deal disappears | the deal remains |
That is why a big percentage in a table is not a prize but a reason to check both sides. The fattest percentages almost always sit where no trades occur.
A quick check before buying to resell: look not at the lowest listing but at the fifth. If the fifth is noticeably dearer than the first, you are buying not at the market price but at the price of the last seller who got tired of waiting. Those are different things, and you will be selling at the third one.