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.
The business market is much more viable. There all sorts of viable markets, like:
- compatibility glue for either server-side or client-side open source alternatives to the Microsoft stack that solves certain headaches (e.g. arcane file formats, better conversion for scripted Excel sheets),
- GUI (system) management tools (such as CPanel and Plesk for budget webhosting, but for less saturated markets like managing thin-clients or virtual machines that are still a moving target),
- hardware + software (e.g. building things like portable barcode scanners for inventory management, security/surveillance hardware or VoIP boxes using the Raspberry Pi),
- et cetera.
If you target business users open source or dual licensing becomes much more viable too. It isn't economical for most companies to dedicate a lot of resources to IT internally, so they will still pay for e.g. support, training or a managed version of your application. The latter is especially attractive with web applications, of course, because you can sell a managed SaaS version of your application (this is exactly my business model with http://www.vixu.com [1]).
0: http://en.wikipedia.org/wiki/List_of_proprietary_software_fo...
1: Based on an Apache licensed application you can find at https://github.com/fmw/vix).