skype is not open source application, so it is not in salix repos. There is a SLKBUILD for skype here. http://people.salixos.org/djemos/skype/SLKBUILD
with fakeroot slkbuild -X can build latest version of skype.
That's because gslapt and sourcery are not the same:
* gslapt accesses our repository with pre-built binary packages
* sourcery accesses repositories with package build scripts