If its something ordered in from none EU then I would aim to start selling them at the end of Sept. That means I would look to order them to arrive mid Sep which gives me 2 weeks just to make sure its all in order and then get listings up. Usually delivery is 1 to 3 weeks so its about now that I start to order that type of items in bulk (although this year I am not selling anything in bulk on eBay)
Most the items (in fact all this year!) I sell though are not bulk items (which in my mind means less than 50 units) and they are from the UK or the EU so generally for non bulk items I don't start selling them until mid Oct which means I start placing orders at the start of Oct
It really does come down to the item though. Some items will sell 10 a week every wee of the year, increasing to 50 items a week at Christmas time. If that is the case I just stick to my regular ordering pattern and just increase the quantity I order around Aug / Sep. Other items will sell exactly the same all year round and no increase is needed.
If its something really small though that takes up very little storage space then anytime of the year really works.
better to have them early than try to hawk them off cheap in the January sales!!! even ****ty products sell well coming up to christmas if the photo is good lol..
Bulk Buying For Christmas 
