One thing from AD&D that we never had happen - not once - was selling a dragon.
We didn't have people fight to subdue, either, which is probably the primary cause of not having dragons to sell.
But overall, it's one of those things I found odd. There isn't much of an economy for monsters. A few - mostly ones that can be trained as mounts - have valuable eggs or young. But you don't have a sale value for orc slaves, owlbears you trapped in a cage, that roper you charmed, etc.
But dragons have a standard market value - and it's not even that terribly high. 1d8 x 100 gp per HP. That huge, ancient red dragon that seems to come up in every campaign somewhere is worth a whopping 88 x 100-800 gp or 8800 gp to 70,400 gp. Not low, really, with an average of 39,600. But how high is that when the average gp value of its monetary hoard is a bit above 80,000?
Potentially it's a way to increase the value by 50%. Or realize profits when you catch the dragon out of its lair and can't find it or get to it.
Yet I've never seen it done. I've read about it in Gary Gygax's columns in Dragon magazine, and in the AD&D books, but never seen it done in play.
Just another element AD&D spent a lot of time on that just didn't impact my gaming.