Unregistered land is now rare and not all conveyancing firms can handle unregistered land sale or purchase. Put simply unregistered land is land which has not yet been formally registered at the http://www.landregistry.gov.uk/" target="_blank">Land Registry. The main reason land remains 'unregistered' is that the property hasn't been sold/mortgaged since 1990, as registration was not compulsory until that time.
As long as you use the right conveyancing form there should be no problems in selling unregistered land provided that all of the original documents are to hand ( these will need to be included in the HIP ). Should you be purchasing a property which is currently unregistered, submission of your application to the Land Registry will now trigger compulsory registration.