member_8880 is correct in stating that you may not REVISE your description unless over 12 hours and no bids. However, you may ADD to your existing description if over 12 hours with bids, or less than 12 hours with no bids.
A subtle difference perhaps, but it may give you an option to starting over.
Ebay rules:
http://pages.ebay.com/help/sell/edit_listing.html