For additional confidence escrows should limit themselves on the amount they hold. They should have a clear public list of the amounts they hold and recommend other escrows when they reach some threshold.
All you need to be sure that the funds in escrow are safe is to ask the escrow provider to combine his key with keys of involved in deal parties (buyer and seller).
That is called a multisig transaction, but then what if the seller and the escrow are the same person? Then that does nothing since he can still scam.