Market Order vs Limit Order

A market order instructs the broker to trade immediately at whatever price the market is offering; a limit order sets the maximum you will pay (buying) or the minimum you will accept (selling) and waits for the market to meet it. The trade-off is certainty of execution against certainty of price: the market order always fills, the limit order always prices. Every ASX retail platform offers both, and the default is usually a market order with a limit option one click away.

The spread is the cost that separates them. A liquid ASX 200 ETF quotes a bid-ask spread of a cent - a rounding error on any parcel. A small-cap quoting a 2 percent spread takes its cut from the market order on the way in and again on the way out, which is the hidden cost the market order pays for the convenience. The liquidity entry covers the spectrum from T+2 to glacial.

The order book is the mechanism. Every ASX stock maintains a list of buy orders (bids) and sell orders (asks) with their prices and sizes; the gap between the best bid and the best ask is the spread. A market order crosses the spread instantly - it buys at the ask or sells at the bid. A limit order joins the book at your price and waits: if the market comes to you, it fills; if it does not, the order expires unfilled, usually at the end of the trading day unless you set a longer expiry.

The spread arithmetic is the second mechanism. A $10,000 market order into a small cap quoting a 2 percent spread costs roughly $200 on the way in and the same again on the way out - $400 of round-trip cost before brokerage. The same order into an ASX 200 ETF with a cent-wide spread costs a few dollars. The liquidity of the stock, not the size of the order, is the variable that decides.

The opening auction is the third mechanism, and the retail trap. Orders placed before 10 AM feed the opening auction, where the opening price is set - a market order at the open accepts whatever price the auction prints, which on a thin stock can gap several percent from the previous close. The limit order is the protection: price it off the previous close and the auction cannot take more than you offered. The broker entry covers the execution plumbing around the order.

For an Australian reader, the order type is the free decision that prices the trade. Brokerage is quoted everywhere; the spread is charged silently inside the price, and the market order is the version that pays it in full. On liquid holdings the difference is rounding; on thin stocks, small caps and thinly traded ETFs the spread is a real cost - a 2 percent spread on a $10,000 parcel is $200 of silent fee each way. The exchange-traded fund entry covers the thin-ETF discount that trades on the same mechanic.

A colleague named Brent (yes, that Brent) market-ordered into a small cap at the opening auction after a favourable news article, and the open gapped 6 percent above the previous close. He paid the gap, the spread, and the article's audience paid it with him. The highest-leverage thing most Australians can do is use limit orders on anything outside the ASX 20, and price them off the previous close rather than the last trade.