I'm not into trucks as collectibles, so I end up with my opinion aligned with Dark Buddha. I would think these are fairly plentiful - or at least parts would be in junkyards.
Sounds like you've done a fair amount of value research already, and you seem to be willing to pay the ebay price. I do think that the '70s Ford pickups are great looking vehicles. I can sure understand wanting one.
One point - if you want a tow/working vehicle, I would think a big block would be superior - might be a good point for negotiating. Also, a 30 year old vehicle with 87,000 miles would probably have a lot of problems from not being used.. dry, leaky gaskets and seals, no hardened valve seats, etc.
My advice is to do your best to wait for him to tell you what he wants. If he wants $700, just pay it and be happy. If he asks for more than you want to pay, tell him what you're willing to pay or slightly less and try to work for something acceptable.
The expression I've heard is: the first person to name a price loses the negotiation.
Maybe this is an obvious point - I'm not trying to insult your intelligence. I suck at negotiation, so I always have to think through everything I'm going to say and do before I arrive so I don't leave too much money on the table.
I hope that helps.