I will preface this by saying I have never taken a 500 apart, but I have taken several other models apart.
It is odd that you would have this many bad displays unless you had some kind of power surge or you are in an environment that has a lot of vibration or is somehow toxic to electronics in other ways. If I was in the mood to mess around, there is usually a ribbon cable that connects the display to the mainboard with a latching connector that allows you to unlock it, slide the ribbon in or out, and then secure it. I would disconnect the ribbon from the mainboard, carefully clean the connections and put it back in - maybe reseat it a couple of times to clean up the connector a bit too. I don't know if you have access to the other end at the display, but if so, do that side too. If that fixes it, then the problem is dirty connections, or the locking mechanism for the ribbon cable was loose or the cable was not fully seated.
I can't explain how it would get that way, but I have seen displays with connection problems occasionally over the years in a variety of products. If you are buying refurbished phones it's possible they were re-assembled on a monday and the person was hung over, it could have happened due to temperature or humidity variations - could be a lot of things.
If cleaning up the connections doesn't help then the display is probably bad or the board itself, but that's an awful lot of bad displays all of a sudden so I would start to wonder how they got that way....