"Value" and "selling price" vary significantly, I've seen.
When I start to research a car or its value, I start with NADA(dot)com (I don't use Kelly BlueBook because they don't have records for older cars I tend to favor
) and find a basic starting point. Then I'll go to Haggerty and find a happy middleground between the two. That takes me 5 minutes.
I'll then spend hours on eBay, Craigs List, Hemmings and local classifieds to get an understanding of what people are actually PAYING for the things.
In the case of the BringaTrailer advert, I'd not go past $6k on that car, and I'm basing that on my own subjective bias (I don't like the interior color, it has rust, etc). Were I to pay for a concourse perfect, every-nut-and-bolt correct, engine-tranny-rear end exquisite, no-miles-having, seriously-zero-rust first place winner at Pebble Beach that was once owned by someone more famous than me, I'd expect to ask $15k for the thing. If it showed up in an episode of Battlestar Galactica and creamed a Triumph or MG, maybe $16k
Not sure why an older one would be worth more - perhaps the algorithm they used biases toward older cars?