Here are some things you can sell to indy devs while they are working: dev tools, hardware, books or online media w/ ads, conferences, certificate programs focused on game dev, marketing opportunities.
Then, when they are done, you get to skim big chunks of their revenue with app store, payment processing, etc. Don't forget to sell additional advertisements, to other game devs, on top of the places where you list their games!
All of the profit streams above scale with the number of devs and the total size of their market - not with the success of the average/median dev.