We're thinking of adding a few pointers to the tutorial screen (or when you open a town for the first time) in the next version to help explain the trading a little better! We've already got the clearer colours and tags underway, so we hope it helps making it a bit more clear! 😊
Inventory and prices change based on several factors. Each town has pre-set price fluctuations for different types of items. These prices are scrambled whenever you get some considerable distance away from the town (since we want to motivate players to trade with different towns). Inventory restocking is based on time and per item, and towns will progressively add stock back over time, up to their original quantity. Hope that provides some insight! 👌