Let's say I'd want to buy a used Mac. To make there are no hardware problems I would run the Apple Hardware Test before purchasing, but how trustworthy is this test? Would there be any hardware problems this test does not spot?
One thing you should be aware of that the previous owner could have spilled water over the keyboard, but the MacBook still is running fine – however, if there ever is a repair under remaining guarantee or a smaller repair which you would be willing to pay, Apple will discover the water damage after opening and not do any repair without you paying a whole logic board replacement etc. (Which in most case will add up to a "total damage" price-wise.)
A faulty GPU chip which has been "reheated" could also fool the Apple hardware test...a major pitfall which you could fall into.
At the end of the day, it depends on how honest the seller is!
Interesting. How would someone go about ''reheating'' a faulty GPU? Would there be any tell tales?
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I've also heard of screen blackouts on modern iMacs. These might not necessarily show up when testing the iMac for a couple minutes. Would there be any ways to provoke these?
As to the question with the provoked blackouts – if you mean the same cause here, you could run a GPU stress test app which will "torture" the GPU to the max. If you're lucky, this could maybe uncover an already failing GPU – however, this would be far from a definite test and as a seller, I would probably tell you that – if you're that suspicious – you should go and try this on the Mac of another seller.
BTW, can you recommend a good GPU stress test app?