All really good comments! I am on the buying side at the moment but the resale value of mods is an interesting subject.
If I understand correctly, a well set up car that is well maintained and not abused on the track, then, although the market might not agree, the car should be as good as an untracked car. But it does seem to me that the car should have more wear per mile than an untracked car.
What questions would you want answered in looking at a tracked car?
If you're making generalizations then this is mostly correct. But there are a lot of 'buts' here. Let's assume 2 cars are both the same year and same miles. One has been tracked a lot, one never. Let's say the track driver is "one of us", meaning he logs everything he does, he maintains things meticulously and constantly inspects and works on the car. Take the second car, say it's owner had a 19 year old kid in the house that got to drive it from time to time. The owner never tracks it and only takes it in for oil changes once or twice a year and that's it. Which one has been driven harder? Well honestly it's hard to say.
Remember these cars are built to be driven hard. And again while your generalizations are fairly accurate with these cars you have to take them on a car by car basis. I'd buy Frank's car before just about any other on the market as I KNOW that he has maintained it perfectly and that it's been constantly monitored and inspected. To me that's a known quantity and I put a lot of value in that.
My advice would be find the right car - and don't worry about it's track history. When you find it learn about the owner, if they did track it do they have a full maintenance log? Mine is pages long as is Frank's. Get an ECU dump if you can that way you know if it's had any overrevs, etc. Have a PPI done, look at the suspension components, look at the glue for the frame. Find out what mods have been done - did the guy slap a 300hp supercharger on it and that's it? Or did it balance the way it was modded, power, brakes, suspension, safety, etc. I owned my car for 3 years before I added power - after I'd done everything else. I think I've modded my car in a very thoughtful and balanced way - and a car like that wouldn't scare me. One that has a few very random mods might.
In the end don't like a "tracked car" scare you. A dedicated track car - sure, stay away. For example you wouldn't want my car for sure as it's a stripped out track toy that's been driven very *hard*, very hard. There's a big difference between a track only car and a car that's been tracked a few times.
I'd start finding what you think is the right car and post it up here - as you can see we'll give you our unbiased view of it and, if the owner is here, we'll most likely know quite a bit about them and the car. If the car has been tracked but they aren't here (or on BritishSpeed) I'd also be concerned as they might not have done all their homework.
Hope that's helpful....