Swade’s theory of used car value

This may turn out to be a pointless distraction, but I had to get it down on ‘paper’ while I was thinking of it.
Yesterday I got a call from the buyer of my former MX-5. He was driving the car up to Newcastle, which is a pretty long trip, and got stuck. The car was running like a dream, which I was always confident it would, but after a stop for some lunch the immobiliser grew a mind of its own.
The immobiliser’s indicator light usually flashes or remains ‘on’ to show that it’s either functioning or disengaged. After their lunch stop, there was no light and no response when he pressed the remote control. That would be OK if it stopped working with the car still being able to start. Unfortunately, it stopped functioning with the car immobilised, hence he was calling me to see if there was another kill-switch or similar device (which there isn’t).
I’m not sure what the outcome was, but I assume they managed to get it going again as I haven’t received another call. It may have cost them some money, though, as they had to get some sort of roadside assistance even before they called me.
All of this brought to mind a theory I’ve had for some time. A theory about used cars. The theory is this:

A used car will be worth X dollars, given its model, age, mileage and condition. If you spend less than X dollars on purchasing this car (i.e. you get a perceived bargain), there is a very high likelihood that you’ll make up the difference in repair or replacement of problem parts.

In the case of the Mazda, I really believe it was worth the $9,500 I had it advertised for when I first started selling it. I dropped the price by $1,000 after I purchased the 99T in order to facilitate a quick sale.
I’ve found that my theory of used cars has an almost karmic quality to it, in that it’s rarely failed to prove itself even with cars in great condition (such as the Mazda). The only car I’ve genuinely got at a cheaper price than what I saw as market value – and haven’t had anything to repair urgently – is my Monte Carlo. And maybe my Alfa 33.
Other than those, I’ve owned about a dozen cars over the years and this theory has always proved true.

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