Hi Marianellen,
If you don't have any luck with the bike shops in Geneva there is a second hand sport shop in Lausanne that buy and sell used bikes (and other sports equipment). I was in there the other day and I know that are looking for more second hand bikes.
Their details can be found at
www.onemoretandem.ch
(and no, I am not associated with the business - apart from spending lots of money there)