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?
Proof is not needed. You just need to convince the escrow agency that it is more likely that you are telling the truth.
What if I send a broken computer, so the purchaser is tricked into signing for it, even after opening it?
Then the buyer complains that the computer was broken. The escrow company (or an arbiter or whatever) decides who is more credible.
What if I send a working computer, but purchaser drops it before funds are released, claims it was broken?
Same thing. The escrow company weighs the evidence and makes a decision or they employ a private arbitrator or whatever. Is it perfect? No. Does it provide some protection to both parties? Yes.