You will need to pu it on a retention certificate before selling.

You will be assigned another mark for the vehicle that identifies its age (this is usually the one which was removed prior to assigning the private plate in th first place, but not always). It obviously helps if it is the original plate as you will probably have the old plates kicking around somewhere and it will save you a few quid in getting new ones made up.

When you sell your plate you can assign therights to the plate to another individual by sending your retention certifcate back to the DVLA with one of the sections completed. Obviously you complete this with the details of the new owner of the plate once you have been paid.

You can try private sale, or one of these websites that trades in them, but they will want their cut of course.

Hope that helps