I think it comes from estates as the older generation passes away. These old bikes are stashed in garages, sheds and barns and as the kids clear the places out of there is no appreciation for the bike (or hundreds of other items...) it goes to charity, many of which will pick up for free. I know the thrift store I bought mine from, we often find "uncommon items" (like 5 Green Bay Packer hats in an Alabama thrift shop ... SCORE!) and the estate explanation always made the most sense to me.

Rashes of old bikes come up on Craigslist every now and then too ... bought 3 oldschool bikes in the past week as projects. No more till I get rid of them

philip