What’s the fastest way to accept payments?

You have an idea. You set up a landing page (you indiehacker, you). But it takes more than some emails signups to get you coding. You want to see commitment. And commitment means cash. As in prepurchases. If this idea wants a vet like you to run it down, it needs to show some sales.

So here’s the Q: What’s the fastest way to accept prepurchases of a course/ebook/service/product that may or may not ever exist?