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Do the proceeds from selling an option immediately convert to buying power in a margin account?
Yes.  I heard back from a couple brokerages that gave detailed responses. Specifically: In a Margin account, there are no SEC trade settlement rules, which means there is no risk of any free ride violations. The SEC has a FAQ page on free-riding, which states that it applies specifically to cash accounts. This led me to dig up the text on Regulation T which gives the "free-riding" rule in §220.8(c), which is titled "90 day freeze". §220.8 is the section on cash accounts. Nothing in the sections on margin accounts mentions such a settlement restriction. From the Wikipedia page on Free Riding, the margin agreement implicitly covers settlement. "Buying Power" doesn't seem to be a Regulation T thing, but it's something that the brokerages that I've seen use to state how much purchasing power a client has. Given the response from the brokerage, above, and my reading of Regulation T and the relevant Wikipedia page, proceeds from the sale of any security in a margin account are available immediately for reinvestment. Settlement is covered implicitly by margin; i.e. it doesn't detract from buying power. Additionally, I have personally been making these types of trades over the last year. In a sub-$25K margin account, proceeds are immediately available. The only thing I still have to look out for is running into the day-trading rules.