Fair market value and willing to pay is more about what feels right to you based on what you know about the condition of the vehicle and what you generally know they trade for. Obviously vehicle specific and bank book dependent.

I think Hagerty is probably a worse source for realistic guidance than BJ / Meecum / etc. based on their focus on high end collector cars. One recent example is a thread on another forum about the price estimates they put on RamChargers. They picked up on one or two extremely nice ones selling for more than top dollar and decided that's where the market for them is now and oddly enough all it's done it's get the facebook clowns excited about how their $8k rig is suddenly an $16k rig. Like that matters.

I think a better approach, though maybe counter to my argument is cost to build. Could I buy a basket case project or decent runner and get it to the level of what this guy is selling for less money than buying his. This being said as I dump more money in to my RC than I could buy a nicer one for and transport it.