Vinted and ebay mostly! try to find sites where people are posting listings that you can pick up yourself (aka in ur town or whatever). If you pick them up yourself they usually give you a better deal. This is how I collected most of mine, if you live in a small area most people selling their old toys wont even know what they are actually worth and they'll just be glad to get rid of it.
If you live in Germany I would 100% recommend using kleinanzeigen :)