I'm about to start building the backend to track my customers, their purchases, create a download area, etc. Why isn't there good software out there to do this? It's a need every single software company has. It's the kind of thing nobody really wants to waste resources building themselves, yet there's nothing out there I've found that seems like a solid solution.

I'd happily pay for this rather than build it. This is not a core business function to me, I want to focus on writing help desk software not ecommerce software.

If anyone knows any good software out there I'd love to see it before I start.