Taken from DVLA site....

Registration certificate V5C
If you have a registration certificate you must complete section 6 'new keeper or new name/new address details' of the V5C. Both you and the buyer must sign section 8 and send the V5C immediately to DVLA, Swansea, SA99 1BA. You are also required to give the buyer the properly completed V5C/2 green section. However, if you sell the vehicle to a 'motor trader' (see notes on back of the V5C/3 for definition) you must complete the V5C/3 yellow section, obtain the details and signature of the motor trader and post it to DVLA, Swansea, SA99 1BD. Both the V5C and V5C/2 (blue and green) sections (uncompleted) should then be passed to the trader. The registration certificate has accompanying guidance notes INS 160. These notes are available for download below.