Ok, say you come across an LCD on ebay, but it says something like"does not power on." Is there a way to make them work or are those just basically junk?

It's basically a gamble. Sometimes it is simply the backlight, which we throw away anyways. Sometimes it may be the circuit board. If you want to gamble bid cheap. That way if it don't work out you're not losing much. Keep in mind that even if it is only the backlight you still don't know if there are FFC issues.