Everything can have it's price sometimes. Throw out a crazy enough offer and most people would take it. I've bought many "Not for sale" items in the past. I've sold multiple keeper items too.

In the end, sale or not regardless of which side of the trade/sale I'm on it usually ends up in a fun conversation because we both obviously share a common interest or enthusiasm for something.

I think your teacher was right. I mentioned that I've sold keepers that I'd have sworn I'd never part with in my lifetime. Situations can occur in people's lives whether it be family/medical/financial that truly make things seem less important. Heck, usually all it takes is a crazy enough price so that someone can trade up for something better (and I see nothing wrong with that).

That being said, I don't think a keeper flag would make a lick of difference. Not one bit.

If somebody wants something, they're going to message you. Hopefully they're polite and you can have a fun time discussing the machine with a fellow enthusiast.