I picked up a Dahon cheap ($100) - then found out why. The grip shifters won't shift down from 6th gear! [I didn't pre-flight check before buying, duh] Disconnecting the derailleur I could get the cable to move with the shifters but once connected to the derailleur it won't budge. I've tried a replacement derailleur and removing the cable from it's routing to stretch it out perfectly straight.
Has anyone seen this behavior? I'm tempted to try a single speed solution.
Another problem is the front wheel went out of true - and broke a spoke immediately.