They’re very easy to transfer, very difficult to sell.
That is to say that if you have a willing buyer at a price you are happy with then great. We’ll process the transfer. Finding yourself a buyer is the challenge. Applicable tax on any sale also applies.
For future reference and to emphasise CC’s response: the company itself has to be involved in the transfer. If they / we aren’t aware it’s taken place and updated the relevant records accordingly it effectively hasn’t taken place (in law).