If you sell something in return for coins, and the network could agree on a fork of history in which you never received those coins, not all your balance is saf

If you sell something in return for coins, and the network could agree on a fork of history in which you never received those coins, not all your balance is safe. Either you trust the network operator(s) not to accept a fork, or you trust the implementation to make a winning fork infeasibly expensive (51% of all miners).