As a long time Linux user, I think this would be very tough. Linux users are used to free software and having everything available through our package managers

As a long time Linux user, I think this would be very tough. Linux users are used to free software and having everything available through our package managers. We generally don't purchase and download software, mess with serial numbers, licenses, copy protection, drm, etc. There is no standard method for delivering software outside a package manager. Anything you do will likely be somewhat hackish and will not work on everyone's system. I rarely ever stumble on Linux software that must be paid for. Off the top of my head I can think of: Mathematica, Sublime Text 2, IntelliJ, and, perhaps soon, the Steam client. These are all obviously multi-platform projects that also happen to be available on Linux (which is nice but probably not wildly profitable). It would help if there were a Linux app-store, but there is not. I think there may be something for Ubuntu along those lines, but the fact that it is only for Ubuntu seriously limits the number of potential customers you have in an already shallow pool. And since Linux desktop users are spread over a wide variety of distros and configurations, a general Linux app-store is unlikely to ever exist.