I recently bought a used Dahon Boardwalk from the German equivalent of Craigslist („Kleinanzeigen“), which works perfectly fine except for the main feature: the folding. It’s somehow stuck and you can hardly get it to fold 90° with one guy holding the bike and the other one pulling. Not very practical if you want to take it in the bus/metro every day. But for the price I thought I’d take the risk.
After going to a bike shops twice the guy told me today to use Ballistol, an oil/chemical initially developed for weapons of the German Army in the first World War. I remember this funny smelling liquid from my army time and it works just as good on bicycles. After trying 4-5 times the folding worked fine again. Same goes for the saddle, which was stuck at an uncomfortable height.