I think beginning of Oct is TOO late! Honest, think about it. You post it on Oct 1st in a 7 day auction. Auction ends Oct 8th. If you are lucky, they pay via paypal and it goes in mail ASAP. If not, you wait a good week for money order in mail. Now we are talking not getting it in the mail before Oct 17th. To be honest, as a parent, I wouldn't want to wait that long to 'mail order' a costume. Does that make sense?
I did a search on ebay for HALLOWEEN; there is TONS of stuff up now and lots of it is selling! I would put it up no later than early Sept.
There, that is my 2 cents.