I had a similar experience a couple of years ago when I was selling a bike.
I had a good idea of how much it was worth anyway, but nonetheless did my research looking at private sellers on Biketrader, eBay and MCN, and pitched it appropriately. I was in no rush as I was using the sale to fund another bike. This total bell-end told me it was over-priced, offered about 60%, and got arsey when I told him he was taking the rise. Sold it about 3 weeks later for the asking price. If he’d have offered 90% I would have probably taken it.