Bank -> Coinbase (user) -> Coinbase (merchant) -> Bank
if user already holds bitcoin
Bitcoin -> Coinbase -> Bank
And now explain the average joe why he should pay for clothes with bitcoin?
He could just go the "normal" way with a simple bank transfer. No risks involved.
With Coinbase he has to trust another entity. It's just more risks without ANY advantages for this specific task.
The user doesn't have to deal with coinbase at all. The user can just send bitcoin to the bitcoin address provided through the Overstock website and that's that. Proof of payment lives forever in the blockchain.