You can revise your listing after you have posted it IF: It is listed as an Auction and has received zero bids, it is a Buy It Now listing and zero items have been sold, or it is a Classified Advertisement.
If your listing does not meet this criteria, you can still add a note to the item description, or add optional listing features. No other revisions can be made however.
You can edit your listing by selecting My Account -> Listings -> Active. Select the listing you want to modify, make changes to available fields, and click the button 'Save Changes'. To change or add images, select the ‘Edit Images’ button at the top of the listing.