I don't know what this fools problem is, as buying & selling should be a simple enough affair.
The protocol is as old as time.

You ask a price that reflects what you think it's worth TO YOU, plus a little bit as a fudge factor.
A buyer has two options. He either buys it at your price, or he makes a reasonable offer.
The seller then has three options. To accept the offer, to decline outright. or make a counter offer
This goes back & forth until an acceptable deal is reached,.. or it isn't, but there is no need for abuse or unpleasantness from a frustrated buyer. It's all so counterproductive, as this dip is finding out.

Datsun 1200 coupes are becoming quite uncommon, while good examples are starting to be downright hard to find. Their market value is reflected in this simple fact.

Bargains are still to be found, so if he doesn't want to pay your price, all he needs to do is keep on looking,....& looking,.....& looking, & shut his mouth.