You can only add a buy it now price after if there have been no bids on the item I believe. Depending on how much the item is, as a seller I would just sell the item privately and remove the advert from ebay once payment was received (as long as you end it before the last 12hours) that way you sell the item for what you want and you won't have to pay ebay completion fee's.. Feedback total wouldn't bother me at all in this instant.. your pretty much going to pay ebay for a single positive feedback