Here is a graph that represents the past two years of sales on eBay in the Sporting Goods>Camping, Hiking, Backpacking> Sleeping Gear> Sleeping Bags> Down category.
The blue lines are the total sold listings, and you can see that in December 06 sales peaked.
May sales are only 45 and 42% of top value for the two years – and that surprises me!
Sales are relatively level for the summer, and last year slowly built up for Christmas.
I may decide to list my item near the Christmas rush, or in March or April, where the sales were approximately 70% of those in December.