I follow BaT auctions. There are enough pictures and feedback that one can make a good assessment of value and content from what's presented. Here is a
listing of recent Typhoon auctions. Put your cursor over a dot on the graph, and a brief synopsis comes up.
These are in the past 18 months:
93 blk/blk 36,000 miles $18.000
93 blk/gry 30,000 miles $28,000
93 blk/blk 99,000 miles $10,500
93 grn/gry 50,000 miles $26,600 (Baythan's former Ty) << Open this link, and look at the pics and comments. This Ty is the best "comp" to your Garnet, and it sold in February this year. (ALL vehicles on BaT do 'better' when presented with stock wheels. The Garnet looks fine with C5 wheels on it, but would command 25% higher price on BaT with stock wheels. "Presentation" shows up in selling prices).
Back in the day when pricing and value was discussed among a much larger group, my observation was that Garnet and Aspen commanded about a 15-20% premium for the color. Royal generated increased interest, but not necessarily a higher price. At 80,000 miles, there isn't going to be any premium for mileage. Condition and Content is everything. In your case, you know the truck well, and that is the attraction for you. You should be willing to pay more than for a sight-unseen that is going to have surprises when it shows up, or you go to look at it. They ALL have surprises. This one won't for you, and this adds value to you.
For the owner, he wants "as much as I can get". In MY opinion, BaT is currently the best representation of "as high as possible" with the least amount of work. Depending on undercarriage, engine compartment, and interior condition, based on the above, I would surmise that an 80,000 mile Ty is going to Private Party (no financing gimmicks) at around $15,000-$18,000. Add 20% for the color and you have $18,000-$21,600. << This seems a little "light" when compared to the grn/gry. In the current market, if that Garnet went on BaT with stock wheels and lighting, and the other 3 areas look as good as the exterior, I think it would go for over $25,000 even with the mileage.
TLDR Version: You are personally familiar with the Ty you want to buy. This is worth a LOT. Be willing to pay more for this knowledge. It's worth it.