You need to be competent enough to know which jobs you can't manage in a timely manner and turn them down. Practically speaking, that means you should be able to write a simple crud app in your sleep. You should be able to sprinkle some fancy JavaScript ui here and there, and you should be comfortable working with APIs. I'd say you should be familiar with payments, but services like Chargify make that so easy, it's no harder than working with Twitter, for example.

If you can do all that without wasting too much time learning, you're more than qualified to freelance.

Note that I said nothing about scaling and other back end magic. Most clients don't need scale. They need front end development.

Be selective at first, and don't take on anything that you can't do well. Referrals are gold, and you get them by doing a great job, not be stretching yourself. That's what side projects are for.

And I would say, always work for full price - period. Again, free is for side projects.