My LBS said they make 80% of their bike sales for the year during April-May and the month leading up to Christmas. April-May is probably a pretty good time to sell a used bike, too. You can't help but notice that beginning just after Christmas and continuing well into January, television and the 'net are inundated with ads for exercise equipment/gym memberships. The first of the year is when everybody thinks about self improvement and weight loss. Bike = more active lifestyle.
It is less fun to ride a bike in January. Have skis for sale?
September and early October is usually a good time for the bike co-op, as college students appear out of nowhere. Perhaps spring too. That may depend on the local organization. Around here, the biggest college towns are only moderate sized cities.