We actually already do this. See this job description:
https://acpol2.army.mil/fasclass/search_fs/search_fs_output....This would generally be responsible for paying parties that actually trust you, using Treasury checks and possibly small amounts of cash, but there is an equivalent position within the uniformed Finance Corps where you pay untrusted parties who also don't trust you. For that purpose, you deliver extremely large briefcases full of cash. Basically, the same idea as a drug deal where the buyer and seller don't trust each other. Everything is transacted in cash, and both parties are carrying a lot of guns. This is generally how we make payments to local warlords in theaters of combat where they are not by any means an internationally recognized legitimate authority you can make any kind of a treaty with, but you need to buy their cooperation anyway.
As for trusting they'll actually deliver what you're buying, again, that's what the guns are for.
Heck, as it stands right now just for normal consumers, buyers and sellers don't need to trust each other. They just need a legal authority who can prosecute each side and they need to trust that legal authority. Barring that, they need an extra legal authority they trust with guns.
As it stands, the obvious limitation of a blockchain is it only helps if your transaction is solely the exchange of data stored on the blockchain itself. Otherwise, you're back to the same issue. The fact that a transaction is recorded on a ledger doesn't mean either party actually received the goods they expected, and it doesn't mean they have any means of remediation, legal or otherwise, if they don't. The fact that a ledger says you own something doesn't mean you're in possession of it, and it doesn't mean the party that is in possession of it has to give it to you.