You can only hide your feedback by making it private, although you won't be able to sell.
To make the items private you need to select it in the listing "Make Private Listing" although that won't make the current feedback private!
you can see the feedback, but the item remains private. I keep all my sales private, but I have well over 1400 positve feedbacks, so the buyers will know I have a pretty good track record.
All my auctions are private too! However like the above poster I have over 5000 feedback so buyers can see I am a solid reliable seller.
I sell collectables in a niche area so buyers like to keep their purchases private from competitors. Some buyers are also resellers and use the stock they buy for a bricks and morter shop in some high end London or New York mall. In fact the only people who have ever suggested there is something shady about totally private auctions are other sellers.
I guess if you are a jiffy bag stuffer who sells thousands of the same thing then privacy is not an issue for your buyers.
hiding feedback for items allready sold 
