Can't improve much on previous suggestions. If you are in the states it seems Craigslist is the most popular "local" free online classifieds for many cities. Kijiji seems more popular in many Canadian cities.
I know where I live some arenas have a bulletin board and some don't. Many that do seem covered up more with corporate crap than private ads (with many of the corporate guys sticking their ads over or removing private ads - I have responded in kind to corporate ads when I see my private ad has been plastered over or removed).
Back to your original question - If your skates are players skates, here's one hockey forum with a classifieds:
http://hockey.inlinewarehouse.com/forumdisplay.php?f=5
There seem to be more forums with classifieds for goalie equipment - goaliestore.com being one of the better known ones.