Another option is to have 2x the delivery fee paid up front, which is enough to disincentivize trolling. Even 1x fee may suffice. The buyer would lose money in a breach of contract AND damage their reputation. Sending money up front massively increases what we might call the Relational Threshold for lack of a better word, which is the sometimes quantifiable damage in terms of discounted future opportunity cost from the change in the reputational state of their key following a breach.

Do you have a way to bootstrap a $500 relational threshold? There could also be a paid relay that has an admission fee that exceeds the fees you want to be able to initially collect as a costly signal. Let's say you want to be able to get $25 order fees. You pay $5 entry to a relay, start with dozens of smaller orders, and organically grow the threshold. The pay to relay, and possibly some payment to relay as a fee on the fee gets Sybil resistant bootstrapping without social graph WoT.

This is exactly why delivery makes sense. The ratio of counterparty risk to transaction value is about 3-20x and final delivery is as close as you can get to a trustless swap. The Relational Threshold is higher in e-commerce.