Yearly timing is hit or miss. Just before the beginning of summer I saw a Caad10 Force w/a new black inc frame go for 900$ on ebay.
The day and time of day the auction ends plays a big part. In my experience auctions that end on a weekend around 10am-12pm Pacific time do the best. More people will be free to participate in the auction and having it end on pacific time will make sure the west has time to wake up and the east will not have started their evening plans yet.
If you do a buy it now always include the OBO option, and AT, or shoot a little higher than what you want. If you shoot too high with the Buy Now option it will deter buyers but if you have a reasonable price with the buy it now, the item usually goes very quickly.
But remember:
1. Between eBay and Paypal you will only take home about 87% of the sale
2. Always put a disclaimer in your listing about how you describe things as best you can and ask all questions before bidding, returned items must be in teh same condition I sent it out in, x/y/z...just so people dont say "you never disclosed this so I want a refund/partial refund"
3. take LOTS of pictures and only put up the best ones on ebay. The more pictures you take the more protection you have against fraudulent buyers.
4. take pictures of the items wrapped up and properly packaged in the box (aka how you packaged it with the stuffing etc.) And take a picture of the outside of the box when it is all packed up. If you don't have these shipping insurance will say "you did not pack it right getting damaged in shipping is not our fault"
5. Never make a deal outside ebay. /discussion.
and finally 6. disclose everything, good or bad and always say "refer to the pictures for further detail" just to cover your butt if you forget something small. This is also why you put up the max amount of pictures and you also TAKE GOOD PICTURES.
But back to your question......the time of year you sell stuff does matter and does not matter, just depends on what you sell. What matters most is how many people participate in your auction. Do your best to have your auction end when people are free to participate in it.