old versus old
When I was selling second-hand and new washers and refrigerators here in Munich, my boss had a simple policy:
If it was old enough or unusual enough for a staff member (and we saw hundreds of appliances every month) to say wow! he'd keep it regardless of the condition - and it worked out, he was always able to sell one of these.
But if something came in, regardless of age, which had the cleaning lady screaming at him or me saying "not on MY sales floor you don't...I am here to MAKE money, not scare away the customers..." he would trash it.
Nothing came into the shop before it had been cleaned up in the courtyard enough to be sure there were no bugs - and that it wouldn't pose a fire risk.
Sure, lots of stuff got tossed which, today, is considered vintage. But we were able to sell great numbers of truly vintage machines, keep our customers happy...and earn enough money to stay in business.
It's always going to be a tough call. But when your livelihood depends on your reputation (and I bet that holds true for all the second-hand dealers on this site) then you just have to see it for what it is: a business.
Oh - we would always hold a machine for a customer who tipped us off that they wanted something, regardless of what we might have thought of it personally. My boss would even put the word out that we were looking for ONE of them. The "you scratch my back, I'll scratch yours" approach works.