Interesting. So in the case of 1, I guess I could have a function in the contract called something like sellUserXStakeTo(userId Y), that would let me transfer m

Interesting. So in the case of 1, I guess I could have a function in the contract called something like sellUserXStakeTo(userId Y), that would let me transfer my 50% ownership to another user Y. And that method would be secured to only accept being called by me, or by another contract written by me. Then if a user Y did come along and wanted to buy my stake, we'd agree a price P, then I would write a second contract that said: If user Y pays user X amount P, then call sellUserXStakeTo(Y). Then, once I'd published it, user Y would verify it, then pay the money, secure in the knowledge that I could not alter or back out of the deal.