In the French-speaking community Ligue des familles organize exchanges in which one can find items for very cheap: https://www.laligue.be/association/activites-mouvement/autres-activites-locales/bourses
In the Flemish community the Gezinsbond organises second-hand exchanges as well. You could also go to Kringloopwinkels.
http://www.gezinsbond-wvl.be/nieuws/detail/118
http://www.dekringwinkel.be/kw/home_1.aspx
Other options are: Brussels Childbirth Trust organises « nearly new sale » : http://bctbelgium.org/social-life/events/nearly-new-sale