Striking deals and power trades
Well, that is self-explanatory really. Demons can swap powers, or steal them. They can even trade them for deeds good or bad, it just depends what they want and where they are getting their powers.
There are demonic markets where all sorts of things are traded. There are also bidding wars, bounties and all sorts of deals that can be made.
Some things come at too high a price, like Fury Talismans. Rare, coveted and theft is punishable by eternal vanquishing, yes they have the power to do that.