If
you transfer your vehicle to a motor trader (see note*), and
you are in possession of a registration certificate (V5C)
you should tell the DVLA of the sale using the V5C/3 section
(yellow) and pass the rest to the trader.
If
you are in possession of the three part registration document
(V5) you should send the V5/3 section (red) to DVLA and pass
the rest to the trader.
If
you are in possession of the two part registration document
you should send the disposal slip V5/1 to DVLA and pass the
rest to the trader.
(*For
this purpose motor trader means motor dealer, a motor vehicle
auctioneer, a motor vehicle insurer with whom you have settled
a claim for total loss, a motor vehicle dismantler (scrap
yard) or a finance company with a financial interest in the
vehicle).