Not sure if this has been addressed..
Location : Semi-detached House
I burrowed undernearth the trader in the middle of the two buildings and got to a small room behind him.
A cupboard [private property] that needs to be lock-picked contains all of his traded wares. I looted everything and left 1 item there.
When i came back the following night or two days later, the cupboard was re-stocked with his items. I left 1 item in there and I could just rinse and repeat the whole process.