Here's a helpful little tip so you don't feel like you've wasted your time openig this thread.
If you're an avid avocado eater like me, you'll probably understand the struggle of bringing home black over ripe avo's.
When in the super market if an avocado seems ripe to the touch simply remove the 'pip' or 'stem' from the top. If it's brown, set it down. If it's green, cash machine.