You can buy and sell almost all second hand baby items (pram, travel bed, potty, shoes, baby toys, activity gym, and clothing at Max und Moritz. They are located at Witikon. www.max-und-moritz.ch/
There is also another second hand shop in Kuesnacht Itschnach. There I've also seen stollers, tricyles, travel beds etc and prices are very reasonable. They don't have an internet site. It is located Im Dorfli, Kuesnacht.