Thanks for taking a look at our game. So in the demo we gave players a stock of 55 items to start, and every day that passes, the Miner will passively earn items that get added to your inventory. We plan to implement the Miner as a playable character so you can earn more items than the Miner passively earns herself. Sometimes the villagers will ask for lots of the same item, and that will cause you to lose all your stock of that item, and your forced to decline the sale and lose that potential money for the day.
Once we get the other characters playable and have our players having a more active role in keeping tabs on their inventory, we hope that it will be much clearer.