He can sell it but it is no use to you until he puts it on to a retention certificate. The DVLA are very helpful if you phone a VRO vehicle registration office. There is a chance they will want to inspect his car but unlikely. I would ask them to get it on a certificate and then you reimburse the fee after. Once on the cert they can sign it over to you.

Seems lazy not to have got it ready to go before advertising it though.

Be careful and make sure it is all done right.