Can Stock Brokers trade against you when they are holding two orders on opposite sides of the market?

External question — Personal Finance Stack Exchange Author: Bob Original post: https://money.stackexchange.com/questions/134287 License: CC BY-SA 4.0 — https://creativecommons.org/licenses/by-sa/4.0/ Adaptation: HTML converted to plain text; contact email addresses removed. Imagine that the XYX (not a real name) stock has a bid of 50.05 and an ask of 50.12. A discount broker currently has two orders. One is a limit buy at 50.11 and the other is a limit sell at 50.06. Can the broker buy the stock from the first client at 50.06 (using the broker's money ) and then immediately resell it to the other client for 50.11?