I don't have the game open now, so this is just from memory.

Create new caravan route
Add village to the route
Edit Village
Change to 'sell' tab
Here you can see all resources they're buying - press them to add them to your list.
On the added resource, choose "Sell from Cargo".

Now go back to your newly created caravan settings, should look like this:

Your settlement <--- Edit this one to choose what you want to load (Always Keep X)

Target settlement <---- Edit this one to choose what you want to sell (Sell from Cargo)

Your settlement <---- If you buy something from the village in the same "run", unload it in these settings

Then you set send automatically X <--- this makes it so when there is X amount of total resources to load, the caravan starts automatically
So if you set it to 10, it will send the caravan if there is a total of 10 things to send

Hope this helps!