How do crypto on ramps guarantee profitability?

I'm not talking about exchanges here, but places that only sell crypto directly with a bank deposit or something. Since they are businesses, they must make sure to profit. But they're always selling crypto, sometimes they have to buy again to restock their supply.

How do they make sure they're not selling low and buying high? Is their 1% fee enough to cover these fluctuations on a long enough timeline?

I mean, there must be a safe way to operate this since there are so many on ramps out there.