if you're the merchant and have Internet, you don't need to trust anyone. the payer can remain offline.
generally, one party having internet is enough to prevent double spending
Discussion
also this is a surprisingly common situation throughout the world
merchant has internet, customer does not
yes, merchant usually doesn't move, customer does!
what about the traveling salesman 👳
the opposite could also exist, tho probably less common:
vending machine (offline) - customer mobile (online)
in that case, customer/sender can create offline verifiable tokens
This was a specific design goal for my Bitcoin Deposits proposal. Always-online vault pays an invoice based on onion message for the invoice and max fee signed by the deposit private key. Potentially a single NFC tap by an offline customer
This is going to take off unbelievably in Africa, we haven't seen anything yet.