First, I agree Larian has to fix this : a message must warn you that you are giving away your stuff for free ...
After placing the items in your column, press the balance icon at the bottom : that trader will then place gold to buy it.
You can also drag stuff the trader is selling onto its column and then click the same balance button.
Here is a short guide you might like or need :
Things you want to know before you play - Basic interactions
https://steamcommunity.com/sharedfiles/filedetails/?id=2250219000