it pretty much works this way:
the club of our gaming group: in the rooms where we play/build & paint models, we have a big box of sprues. When we build something, we put there the remnnants and it's free reign for everyone to pick bits for conversions and so on.
our gaming shop: there are 3-4 bowls with many unused bits... many of them are the remnants of boxes of sceneries built by the owners for gaming use; some come from commissions ("i pick this box of necron immortals, please build and paint them with tesla carbines"); if you're lucky, you can find the pieces of whole models that were left unsold after the selling of individual units from battle boxes.
many of those bits (even some good ones) can stay in the bowls for months.
I even found the pieces to build an almost complete Drop Pod, missing only the internal parts.