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.