Steve Preston, 51, owner of Sergeants Towing in Portland, Oregon, on his 1944 Buick “Hellcat” M18 tank destroyer, as told to A.J. Baime.

Once you’re a collector, you’re always a collector. But your passions can change. As a kid, I collected knives. Later, I got into military history, so I started collecting military knives, then guns, then eventually, vehicles. Now—though I never served in the military—I have about a dozen military vehicles, and most of them run.

Everyone...