Why online brokerages sometimes allow free ETF buying
The same reason a company would offer coupons. I'd guess they're just doing it as a way to entice people to do their investing with them. Since it is any ETF I doubt they are being compensated by the ETF companies, as is sometimes the case (iShares does this with Fidelity, for example). And they still get the commission on the sale.
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Why online brokerages sometimes allow free ETF buying

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The same reason a company would offer coupons. I'd guess they're just doing it as a way to entice people to do their investing with them. Since it is any ETF I doubt they are being compensated by the ETF companies, as is sometimes the case (iShares does this with Fidelity, for example). And they still get the commission on the sale.