i have reported this to Ebay. There have been similar listings in the past where a piece of tube with a frame number stamped in has been offered along with a 'matching' log book. The suggestion in this case is that you could use this logbook (registration number) and the frame number to 'manufacture' a pre 1965 trials bike. Using the logbook / frame number in that way would be illegal. In fact using the logbook/ registration number other than with the original frame is illegal.
The seller clearly does not know that a leader / arrow does not have a stamped in number or in fact a tubular headstock. Whether Ebay understand the legality issues is another matter.