Spoke to the DVLA about this earlier and explained that i was in the middle of selling the car and the lady said that the buyer could use the section 10 "New keepers details" stub to tax the car at the local DVLA only (not a post office) and then when the V5 comes through i could fill in the guys details as normal and he would then get a new V5 in due course. I told the buyer this and he said he is happy to wait a few more days to see if the V5 comes through and he will then take the car as normal so hopefully within a week the car will be gone.