About your point 'set up a service for these people to sell their goods to Americans.' - Never thought about this. So you are saying that there be dedicated businesses selling to americans, and 'shipped by' people who are travelling? Have I understood your view correctly?
You would handle things like payment and verifying the identity of your buyers and sellers, but the actual procurement and shipment of goods would be handled by the seller/entrepreneur.
I have a friend who ships handmade handbags from Senegal and sells them in NYC in the summer. She's responsible for dealing with the production issues in Senegal (not fun) and the shipment to NYC.
Now your service would be to offer this person a popular online space to sell the bags to a broader audience.
*It looks like what you are thinking of is for the person doing the traveling to basically buy the stuff and bring it back or vice verse take it with them?...
Yes Exactly.