Lets face it. Apple does not allow many very cool apps for sale in the App Store because of their very tight (and in many cases unreasonable) restrictions. The developers of these apps (PDANet, MySMS and iBlacklist to name a few of the many) still want to earn a little revenue for their hard work and until now have been forced to come up with their own sales solutions on Cydia/Installer.
Tonight the game will start changing as Jay Freeman (Saurik) releases the Cydia Store. Just moments ago Freeman announced on Twitter that "Tonight (late) a new Cydia will come out with one package 'for sale' (more over the next week): more details will be in Cydia."