As far as I understand they get it for free...selling it for charity.
Yes they get it for free but I'm not all that sure about the charity part. Some of these Kringloopwinkels have turned into complete businesses where the lines between charity and profit seem to be getting smaller and smaller....
They also seriously hurt initiatives like our yearly flea-market in which we try to earn some cash for the club...we get less (good) stuff offered because of them.
I've only been to one in Utrecht and usually the indicated prices are completely nuts.