It's a little early in the season, but Craigslist and Kijiji will be filled with ads soon.
Around here, there are tons of company/department ski clubs that organize discounted group buys for various places in the pre-season.
Around late Feb/early Mar, lots of people realize that they've only used 2 or 3 of the 8 passes they bought and the season will probably end before they can use the rest. They often unload them for face value. After March break, you'll see people selling them in the parking lot.