The classified section of Glocals, under "goods for sale"/"sporting equipment" is worth checking, as bikes come up every few days.  For an even bigger selection, check anibis.com.  Lots and lots of bikes for sale there, and fairly good deals can be found.  I just got a nice, used bike from there recently.

Otherwise, there are a couple of shops in Geneva that also sell used bikes, but I've found that better deals are to be had if you are patient and keep checking online. If you want names, let me know.  (as it will take me a minute to find out what they are all called, I only know them by location)

Good luck!