If you have items that someone wants to buy it doesn't matter what month you list it. It will sell if it is not priced too high. The Ebay seller fees and shipping are going to cost the same no matter what month you sell it.
One thing I can say, that saves me time and money when selling on Ebay and other sites is, I print my shipping online, either through Paypal or directly through USPS. Usually right from the Paypal website, and doing it online you get free delivery confirmation and free postal carrier pick up, as long as you are shipping Priority, which I do. It's just a few pennies more than Parcel Post but 4 times faster shipping. I save a few bucks every time I don't have to drive 12 miles to the post office and back.