You sell it. And users are happy to pay because they're getting time and value in return. I also sell a product - not something database related but it doesn't matter. The users get value out of our product. To get the same thing elsewhere it would either cost them much more, be a lot less flexibly available, or be of lower quality.

If you were a gardener would you also find it hard to make people pay for you going to their house and taking care of their garden? They're paying for it because they feel it's worth it.

Of course, it depends on what you're selling, but I'm assuming you're not asking about selling gambling products or crack cocaine. As for those, the answer is "be a psychopath".