I know that when people really want a game, they'll sometimes contact random people that own that game. I know on pinballowners.com they give three options:
1. Never for sale
2. Possibly for sale
3. For sale
That way if someone searches for pinballs anywhere in the world, they can see who wants to sell, or would possibly entertain offers. Might be nice to also have this feature on pinside.
Obviously if it's for sale, you'll just throw it in marketplace
If it's nailed to the ground, you want people to know that you don't want to even be contacted.
If you're on the fence (you aren't necessarily looking to sell, but might sell for the right offer), then there would be an option to tag it as such.