I picked up a pair of Joycards for 100 yen each. On one, the A button doesn't respond. On the other, neither the A or B buttons respond. Any thoughts on how/if this could be repaired?
You might try to replace the AB conduction (rubber) pad. This pad should be similar to nes/fc conduction pads, so it would be easy to find a replacement. And don't forget to clean the joypad pcb with some isopropyl alcohol.
Basically start off by cleaning the board with alcohol, and then clean all the plastic and rubber pieces in warm water with soap. Also check to see if the rubber parts have been ripped. I've cleaned a few Hudson Joycards in my day ;)
If you are a hardcore soldering fan you can replace rubber conductors with SMD tact switches.