My friend like to talk about his first Alfa buy. He joined the local chapter for a few reasons- one being the motorsports aspect. After a short while of looking, he found a deal on a GTA Jr in the north east, and contacted the dealer (yes, a dealer) and assuming the car was ok, it would be traded for a decent Datsun 2000.
Now, a Datsun 2000 is a fine car, but the GTA Jr is a homologated car that is largely a aluminum bodied special, and it had a very special short stroke, twin plug 1300 that made somewhere near 125hp, in street trim. It was made for sedan racing in Europe, and very, very few street cars ever made it to the US that didn't go racing quickly (lots of them did TA and C-Sedan racing). So, yes, the 2000 is a fine car, and very collectible, but nothing near the GTA.
So my friend, and another friend who was at the time more of an Alfa specialist jump into the 2000 and drive to the north east to see this car. Get there, car is in good shape, start it- it's making a horrible sound in the front of the engine. My friend seems concerned, but friend 2 whispers "don't worry about that..."
Bad sounding GTA Jr is traded straight up for decent 2000 (and at the time, it was just another used car- this is the early 70's, but the Jr was still more valuable). Friend 2 asks dealer if they can jump into the garage and borrow a 14mm wrench. Start car, turn to loosen chain tensioner, turn to tighten, sound immediately goes away, runs great, dealer's mouth hits floor. His ignorance cost some serious $$ on that deal.
My friend has been agressively driving that car for close to 40 years now.... Probably worth 5-10x that of the 2000. And a very, very nice car to drive.