They make money on the margin accounts when you borrow. They also make money on loaning out the stock in your account. That is a very profitable portion of the business. The actual cost of doing trades via computer has dropped so much since 10-20 years ago. When I used to be in the brokerage business we paid 8.00 for a ticket to our clearing agent. Now the incremental cost to do a trade electronically must be tiny.
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