The seller wants to sell you an aeroplane. However you will have certain requirements that you will want your aeroplane to fulfil.
A good company demo pilot should sit down with you, discuss your requirements and how well his product can fulfil them. Whether you can log the time P1 depends on whether you have the class rating on your licence and any appropriate differences training, and the owner is happy for you to sign the tech log as pilot in command.
However, if the company pilot holds an instructor rating you could log p u/t if you are not qualified. If neither then you are a passenger.
I would suggest a thorough brief as to what you want to accomplish from the flight, and who will do what.
Hope this helps.