Charging someone to demo gear that they are selling is insane! The seller should accommodate you in every way possible, including sharing his or her expertise. There are many times when certain components do not pair well. On a different note the seller should ascertain the seriousness of the buyer if it is a private demo. I do private demos but I also ask questions about the gear the person has. If the person has a Marantz receiver and wants to demo some MTRX/2 I would have reservations.