As a rule for myself, anything under 6 traits I ususally do not sell unless it has a special quality (new trait or very recessive trait), and even then I sometimes opt to keep them and get that trait to pass (like now I am fighting with blonde streaked whiskers).
So 7-9 trait kitties with midrange eyes, and mysterious traits I will sell for 600-1000 linden. 100L per trait is usually a pretty fair starting point.
Kitties with upper range traits, very recessive traits, retired furs I will sell at auction. I suggest browsing around other areas and see what similiar kitties are selling for. Market research is always the best
Good luck and have fun!