Filled at the bid, down 8 cents a second later, 0% fee: what queue priority and adverse selection mean for KCEX spot limit orders

KCEX's guide starts with a made-up but very familiar trade. Mid price 100.00. Best bid 99.98, best ask 100.02. You rest a buy at 99.98, it fills, it looks like you bought under the mid. Then the mid drops to 99.90. You are down 0.08 per unit immediately, and the 0% commission did nothing to stop it.

The image is a live logged-out capture of KCEX's BTC/USDT spot screen taken this morning: the order book with asks starting at 81,396.06 and bids at 81,396.05, a Limit buy panel, and the fee line showing Maker 0% / Taker 0%. Those are capture values, not live prices.

Your limit order is a price, not a promise

A limit buy says the most you will pay. It does not oblige anyone to sell to you. If lots of orders already sit at the same bid, yours joins the back of the line, and matching engines generally fill by price first, then time. The market can trade exactly at your price and you still get nothing because the selling volume ran out before your turn.

Adverse selection: the fills you get are not random

Resting orders tend to get hit hardest right before the price moves against them, because whoever is selling into your bid may know or react to something first. That is the 99.98 -> 99.90 example. KCEX's guide points to the metric that catches it: the post-trade markout, i.e. where the mid is a short time after your fill. If your passive buys keep getting followed by a lower mid, the spread you think you are capturing is being given back.

Maker vs taker is about urgency, not about who is smarter

With both rates at 0% on an eligible pair, the fee difference disappears, but the trade-off stays. Taking means paying the spread to be sure you are in. Making means a better price if you get filled, with the risk that you don't. If you expect the move after your signal to be much bigger than the spread, waiting can cost more than crossing.

Partial fills and cancel/replace

• Order 1,000 units, get 200, and price walks away. Chasing the other 800 can leave you with a worse average than buying at once; cancelling leaves you under-positioned.
• Cancelling and re-placing to follow the price can reset your time priority. You end up at the back of a new line and may still not fill.

The candle myth

A one-minute candle that touched your limit does not prove your order would have filled. It says nothing about how much size was ahead of you in the queue. Backtests that assume every touched limit fills look better than reality. KCEX suggests conservative assumptions instead, for example only counting a fill if price trades through your level.

What to actually log

Decision time, limit price, size, fill time and quantity, the mid before the order, the mid a short time after, and what happened to the unfilled part. A few weeks of that shows whether your resting orders earn the spread or mostly get picked off.

The KCEX notice behind the 0% rate describes it as a promotion for eligible spot pairs with the end date to be announced, so check the pair before you count on it.

Rule of thumb from the piece: zero commission removes one cost, not the cost of being filled at the wrong moment.