I don't sell much in the way of cycle things on eBay, but I do sell motorbikes and Spring is the thing. Autumn is ok for winter bikes and for projects, people tend to want projects for winter.
Cycling being weather related too, that would suggest people might indeed be looking for parts during winter if they have projects on the go.
I tend to 'clear things out' in autumn too, (far too busy in the good weather to be doing that), so I guess that might result in people listing things... but the more people listing the lower the prices, right?
I have sold a lot of amps and guitars, these tend to sell all year round.
Back in the day I bought and sold camper vans, and the rule was buy in autumn, sell in spring. Good money to be made.
Perhaps a poll would be helpful?
Interesting question anyway!