know companies which buys?

You can look up "Service vide maisons" in the yellow pages, www.pagesdor.be

Alternatively, charities will take your stuff as well, such as the Salvation Army, the Red Cross, Les Petit Riens etc.

Trots sell stuff but have to take it in and then they pay you once it's sold. You also have to call ahead and get the stuff ther. Troc.com has a list