Any online site that sells bitcoins for fiat, like Coinbase, could just keep the fiat (or use it for anything other than obtaining bitcoins) and increment your

Any online site that sells bitcoins for fiat, like Coinbase, could just keep the fiat (or use it for anything other than obtaining bitcoins) and increment your bitcoin balance. Their total bitcoin holding may be less than the sum of their users’ balances, and no one would notice unlrsss there was a “run” of users attempting to withdraw bitcoins into their own real wallets. We’d probably all consider that blatant fraud.