Dunno about overstock but when i'm buying humble indie bundles with bitcoin, they use coinbase too and i have no coinbase account. I just scan the QR code using my android bitcoin wallet (called mycelium) and tap 'send'. No confirmations needed, beause it's small ammounts. Maybe overstock will need some confirmations for larger purchases.

So yes, there is actual bitcoin transaction going on if you don't have coinbase account.