The "once someone is paid, they stay paid" is a feature of BTC. It would be nice if there were an escrow mechanism with a time limit so if Alice sells a vend a goat machine to Bob, Bob puts the BTC in escrow, until Charlie vets that the vend a goat machine made it to Bob's place and is usable, then allows the transaction to proceed, or before a time limit, interrupts the transaction and has the money sent back to Bob if instead of a vend a goat machine, it were just a box of cinderblocks. This will help against one of the more common auction frauds, and it protects the seller (the currency goes into escrow before the product is shipped), and the buyer (the escrow agent validates that they actually got what was in the package.)
Of course, this isn't perfect... the Bob the Buyer can pull the vending machine out, place some stones, then allege fraud to Charlie so Charlie nixes the transaction... but that goes from common auction fraud which is an everday happening, to actual felony larceny. Escrow does raise the bar though, and given a high enough value transaction, it might be Charlie has his people waiting with Bob for the package to actively validate that all shipped as it should have.
There is another downside... Charlie's reputation. This was discussed back in the 90s on the cypherpunks list, that if the value of Charlie's reputation was less than what the transaction was, he could collude with either Alice or Bob to fuck over the other party. It might sully Charlie's doings in the future, but if the transaction was valuable enough, hosing one party might just be worth it to the escrow agent, as they could go find another biz after that.