No one can possibly predict the right time to sell Post Office shares. Looking back, other privatisations have seen shares trade at a modest premium to their initial price. The best thing is to decide the risk/reward based on holding onto your shares and decide if you can take the risk. Those selling in the first few months of privatisation have seen their shares make a significant profit non their shares.
If you bought Post Office shares through the public Share Offer then how you sell them depends on if you chose to hold them in their Nominee Service or to receive a share certificate. If held in the Nominee service, then you can sell them via that service, or take them out as a certificate and sell the shares through a broker of your choice, or alternatively transfer them to a broker to sell post office shars on your behalf.
If you have a share certificate then you will need to open an account with a low cost execution only broker to sell them, or they can be sold through the Registrars.
If you bought your Post Office shares subsequent to the initial public offering through a broker then you can sell though the same broker.
NOTE: This website is for information only and is not intended to give advice. You should check out the information with a Stockbroker or your IFA.