They already do restock some items. For example, innkeeper restocks some food and alcohol. Valdemar restocks nails and Roman restocks gasoline if you have the motorcycle.

They don't restock everything, since I want some items to be limited, but items you need in large quantities like bowls, bottles and nails do get restocked.

Again, this is unfinished. I will be overhauling their inventories as the game progresses and more settlements and traders are added, which means I will make them restock more items.