I have learned that on a non-Yankees week night with iffy weather, there are many many people selling Red Sox tickets, right outside of Fenway Park. If the Celtics or Bruins are playing the same night, this dramatically increases the number of face-value ticket sellers.
Jane's Lessons Learned 
Can you count on it? So if you spend time and money to get to Fenway, how likely is it that you’ll see a game?