The available cash in a brokerage account can be a bit of a fiction, in that the brokerage may not literally hold it as cash, they may put it in a low-yield saf

The available cash in a brokerage account can be a bit of a fiction, in that the brokerage may not literally hold it as cash, they may put it in a low-yield safe investment and keep the interest, and they may let you trade with unsettled funds from stock sales or bank transfers that haven't cleared yet. US stocks have T+2 settlement, when you buy or sell the money and stock are really only exchanged two days later by the clearinghouse. Until that happens, if they let you trade the proceeds immediately, you're kind of trading with a very short term loan that has your "sold" stock as collateral, it's technically not totally risk-free. You can buy with unsettled cash even in a cash account but there are a few rules to follow to avoid violations. In a margin account you may be trading with margin without realizing if you trade frequently and don't pay attention to settlement. Even if you're not trading margin the whole system is kind of running on short-term credit and there are capital requirements. The clearinghouse has to make sure the brokerage can fulfill all its obligations for all the unsettled stuff for the next N days even if it goes under tomorrow. (EDIT: The whole financial system tends to run like this.. you can spend the money from a deposited check long before it has cleared, etc.)