Isn't it like you buying a product at a general store, say it was a $10 box of crackers. You then go to a friend, and sell that box of crackers for $10. The store gains $10, and isn't getting ripped off. The only person who is losing money is the person buying the box of crackers.
At least that's what I think, I'm getting confused by how the third party seller is ripping off Blockland.