The bid is one half of the quoted spread. Placing a market sell order fills at (or very near) the current bid.
The bid-ask spread is effectively the round-trip cost of trading. On the largest US stocks that gap is often a single penny; on tiny stocks or after-hours it can be several percent.
Practice this in Aurora
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Open Decision Lab →Frequently asked questions
What if there is no bid?
Rare on major exchanges during regular hours. It happens most often in pre-market, after-hours, or in illiquid securities — signalling caution.