Out of curiosity, how would an escrow agent work against malicious actors (without the law serving as a deterrent, since enforcement against online fraud is near non-existent)?

Scammers are already tricking PayPal's dispute system by sending real tracking numbers and sometimes even real packages but filled with bricks or other junk.

Imagine a situation where the buyer is malicious and claims they have received a brick. If you settle in favour of the buyer, sellers lose out, but if you settle in favour of the seller, buyers would lose out from scam sellers sending bricks instead of the promised goods.

A neutral party such as the shipping courier would have to act as a witness and unpack the goods on delivery to mitigate that, and even then it's not bulletproof if the goods have a defect that isn't immediately obvious.