I'm pretty good about not buying them on a whim - though I'm really bad at trying to talk DH out of a machine he wants to get mostly because it's cheap. He has delusions he'll fix them up and sell them - he may fix them up, but he really does not like the selling part.
The simple truth is there are way more machines than people interested in them. So no matter how many any of us buy, next week/next month there will be more on the market.
I figure, if we buy them, that gets others interested in cleaning out the closet or storage shed - and who knows if they may find a real rare gem! I've been heartbroken to learn that someone who didn't know one machine from another junked out (or made into a lamp) a truly rare machine, because they figured no one would want it.
The real problem is storage - because the people like us who do buy them are not all that common.