Thank you for the opportunity to help you with your question!
Think of inventory in terms of time.
Have at hand only enough inventory at a given time to support planned sales, until the next delivery. A retailer should also have a safety stock to cover spike in short term or the possibility will be late on his delivery.
A retailer can have the number of weeks of planned sales going foward that is represented in the current inventory
Please let me know if you need any clarification. I'm always happy to answer your questions.