I run a video games business and fix button issues all the time! All you'll need to do to fix a GBC with stuff buttons is open it up (requires a tri wing screw

I run a video games business and fix button issues all the time! All you'll need to do to fix a GBC with stuff buttons is open it up (requires a tri wing screwdriver, $2 from eBay), clean the button pads with a q-tip and alcohol. The pads in question are the black bits on the back of the rubber immediately underneath the button, as well as the circular copper traces on the board that the black bits come into contact with when you depress the button. You might need 3 or 4 q-tips. You'll know when you're done because the q-tips will stop turning black. Takes 10 minutes overall and is only slightly more technical than wiping your arse! Don't be intimidated!