I know basically how escrow works, but I don't understand how it would really remove risk.

Say I want to sell an iMac computer. It is worth a fair amount and I am a relative newb on here so somebody is going to want escrow.

So say I sell the iMac for 10 bitcoins. The purchaser sends 10 bitcoins to the escrow service, I send the computer.

The two risks I can't figure out are:
What if instead of sending a computer, I send a box of books?

Buyer tells escrow agent and I don't get bitcoins.

But what if I did send the computer, but the purchaser claims that it was just a box of books? How do I prove otherwise?

What if I send a broken computer, so the purchaser is tricked into signing for it, even after opening it?

What if I send a working computer, but purchaser drops it before funds are released, claims it was broken?

Thanks.