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How do brokers make money from margin accounts?
Your broker will charge you commissions and debit interest on your "overdraft" of $30,000. However it is very likely that your contract with the broker also contains a rehypothecation clause which allows your broker to use your assets. Typically, with a debt of $30,000, they would probably be entitled to use $45-60,000 of your stocks. In short, that means that they would be allowed to "borrow" the stocks you just bought from your account and either lend them to other clients or pledge them as collateral with a bank and receive interest. In both cases they will make money with your stocks. See for example clause #14 of this typical broker's client agreement. Applied to your example: In other words they will make $60 + $450 + $1,800 = $2,310 the first year. If the stock is expensive to borrow and they manage to lend it, they will make a lot more. There are by the way a few important consequences: