I'm not at the culling stage yet, but I have been trading things out recently, and I think, as others have said, a lot of my enjoyment is in building the bikes up. So selling/buying a complete bike has almost zero appeal.
I'm always going to want to keep certain things off a bike I sell, for other projects (even a new bike looks better with a few, choice, old bits!) And other people are always going to want to do certain things to any bike they buy from you. Varying tastes being what they are.
Then you factor in the losses from the cost of parts purchased vs a realistic price for a full bike, and it starts to make almost no sense selling something complete (unless, maybe, it's a catalog-spec, garage queen unicorn)
It's not that I don't love riding them, I do, but, let's face it, the differences between most of our old mountain bikes are negligible at best. In the sense that if they're good for one type of riding, the others of their kind probably will suffice too.
So then the question becomes: Do I care more about owning a bunch of different old bikes because they mean something to me; because they're important in their own ways? Or do I care more about owning different bikes that ride differently and can be used for different things?
Because, let's be honest, we're all owning multiple bikes come what may! There's no way any of us are getting down to one. If we were we wouldn't be here!
But I can see a future where I own a few less vintage 26" MTBs, and a wider variety of bikes, new and old, in general.
Problem is, I can just as easily see a future where I own 100 vintage 26" MTBs and nothing else!
What am I saying?
I guess: Goodluck!