Let's see... According to twitterholic the 1000th most-followed person on twitter has 75,000 followers. That means the 10,000th most-followed person probably has 7,500 followers, and the 100,000th most-followed person probably has 750 followers. This means the market for this product probably caps out at 10,000 or so users who both have enough followers to really move the needle and also something lots of people would want to buy. And that's probably including Facebook, LinkedIN, G+, etc. as sales vectors also.

I love the idea behind gumroad in theory, but the problem is that now not only do I need a great product but I need my own audience. This seems like one step forwards two steps backwards.

That's not to say suggest that gumroad is on the wrong track, especially since there are a lot of things you can do with this, but on its own I tend to be extremely skeptical of any business that relies on their customers already having lots of engaged followers.