Collecting and mining hands you play\see is fine but its the buying and selling of other people's datasets that is bad form / banned. In the small circles of nose-bleed games, it is resembles collusion when a team shares data to undermine a single target across a set of games.
It's still pretty standard to mine hands that you didn't personally participate in. Lots of players mine nonstop. You're right that you're probably crossing the line when you buy/share other people's datasets, as you'll get information that your opponent can't possibly get without also sharing/buying datasets. That being said, of the nose-bleed players I personally knew it was still the norm to share data with your friends. I think at that level you just have to assume that all of your previously played hands are visible to all your opponents and adjust accordingly.