The trading system wasn't implemented yet in the D:OS builds shown during the kickstarter.
In DD the inventory reset every 10 to 15 minutes (depending on the NPC), though merchants could hold on to some items you sold them. In Divinity 2 there was a fixed gold amount for each merchant, and possibly pre-set items, and the rest reset each time you went up a character level.
In DD merchants didn't get richer as a result of trading with you (their opinion of you went up, and they gave you better prices), but that could be an interesting mechanic to see in D:OS. Even without more gold directly, they could acknowledge the increased business in dialog options, hire a someone to help and increase the hours a shop is open, make visible improvements to the shop itself (or move from a cart to a shop, or from the outskirts of town to a more central location). In lower population areas, maybe merchants could comment about still not having sold the last stuff you sold them if you trade with them over a certain amount.
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