the problem as i see it, boils down to… that the trade needs to be verified by a 3rd party…
somebody to hold the bag of gold and verify the item being traded is what is claimed.
without some physical institution, person or device in the middle either party can easily trick the other because of the distance and delay in the trade.
if someone sends money, then the seller might not send the item…
if the buyer won’t pay until the item is received then the buyer might not pay.
if either party or a 3rd party has a trigger or lock that releases the money to the seller after the buyer has gotten his item, then the buyer can claim the item was broken, or the seller could have sold a broken item without anyone knowing it…
so one ends up with one solution that removes most of these issues… tho no solution is perfect.
a 3rd trusted party needs to hold the money and verify the item and then do the trade, i’m sure smart contracts could be involved with that…
it all boils down to trust… from an objective observers point of view, neither the seller nor the buyer can be trusted, because one cannot get enough information to determine if anything they say is true.
if the buyer’s payment and the seller’s item is true… thus the deal can be verified and commence.
ofc that system also have some drawbacks… mainly the 3rd trusted party needs to be able to verify the item is what it’s suppose to be and know that it is functional, but sure is a lot more difficult to cheat in such a setup…
alternatively one might make some sort of deposit, crypto locked trust value using some sort of MAD contract like solution… Mutual Assured Destruction…
if the trust is broken and somebody pushes the big red button everybody involved losses…
much less labor intensive, not sure well that would work either… people tend to push red buttons lol
i suppose one might do something kind a public key scheme…
both pay a deposit based on some value of the trade into a MAD smart contract of which either party if dissatisfied could destroy.
and then after a certain time lock it will be released again if nobody was MAD enough to push the button.
really the only thing one is doing is trying to mitigate scams, somebody will eventually find a loop hole…
here we have a commonly used system with verified identity… that way you can know who you are dealing with at the very least… keeps people from making thousands of accounts and scamming people, seems to work quite well…
ofc small country so i suppose it’s much more likely that if you scam people they will show up on your door step… 
sorry that i don’t have an easy answer for you…
i’m sure smart contracts can be used in many solutions in regard to something like a 3rd party trade…
i will say tho i don’t think the 3rd party trust deal thing will be efficient enough for what we need to do here… so we will need a similar safe guard, that doesn’t involve sending items around to multiple places.
it’s just not easy to do “securely” without something like that…