If you have the right screwdiver, or are able to build one yourself (-> <http://www.projectvb.vze.com>), you should open your VB and see if all cables are connected properly, maybe one to the left mirror is a bit loose, which is a common problem.
But maybe the VB had been dropped by its former owner and the mirrors inside have become out of alignment. If that’s the case, the Nintendo Consumer Service would be your only chance, but I guess that would be more expensive than just getting a new one… I suggest you do the alignment check which is described on the manual of your VB.
Hope that helps.
I had the very same problem and DogP advised me to disasemble the VB and check to see that all of the ribbons were connected securely. It fixed the problem for me, hope you have the same kind of luck.