Hire a business broker. To get the highest and best price you need someone with experience in buying/selling businesses.
I think what you are actually meaning is that your friend has a Newsagents business that incorporates a sub post office. Your friend needs to talk to her Royal Mail contact, as far as i am aware she cannot 'sell' the Post Office portion of the business as such.
Put a card in the window.
I sold my UK catering business through daltonsbusiness.com.
my friend has a post office business which incorporate a newsagent. what would be the best way and quickest cheapest option of doing this.