Offer him half of what he's asking and never go over 3/4 what he's asking. You could get home and it not work and there would be nothing you could do. I don't ever buy used electronics unless it's vintage and repairable which most old stuff is before microprocessor tech. I have been doing car audio for over 18 years and digger is right, salt water is the worst. I hate low balling but if they are not being 100% honest it's the way to go. Also have someone go with you or let other people know.