I can't understand why you're so sure about that. First of all, every person in the world spends money. The first dollar identifies a customer interested in your product, absolutely. But that means your product, not any random product in the world. So it's not a signal at all for being a good advertising target.

That's like thinking that a girl is likely to sleep with everybody because she has a boyfriend. "She's already sleeping with one guy, so why shouldn't she sleep with everybody?".

Of course your current customers are excellent targets for your own upsells and your other products. But not much else.