When I was setting up Primate Labs as a proper software company, one of the things I enjoyed the least was setting up the online store. Figuring out which payment processor to use in the first place was challenging enough, but integrating it with the online store was downright tricky! Of course, if I was setting up a store now, I'd take a look at Potion Store, Potion Factory's open source online software store, um, software:
Potion Store is an open source Ruby on Rails application that powers the Potion Factory Store. The goal is to help new Mac OS X developers get up and running fast without having to sign up with services such as Kagi or Esellerate. The only thing you need to bring is your own license key generator and a PayPal or Google Checkout merchant account.