A possible way could be to track using locations. 

1. Create a location for each reseller

Partners/ Customers/ Reseller 1, Partners/ Customers/ Reseller 2, Partners/ Customers/ Reseller 3 etc.

2. Create an internal transfer from Partner/ Customer to these individual locations. Include all the products with maybe Qty as 1 (so they are visible in the report)

3. Navigate to Inventory > Reports > Locations .. modify the default filter to Location > Location Type = Customer Location (default is set to 'Internal Locations') and group by Location. You can view as list view or Pivot

4. To modify the product qty at each location, switch to list view and edit directly the On Hand Quantity. By clicking History the inventory adjustment can be verified.

Additionally you might want to associate these locations to the resellers. Navigate to Contacts App, Sales and Purchase Tab and update the Customer location field in the inventory section. (Note this section is visible only in developer mode).

This would ensure that any sales to the reseller automatically updates the associate inventory location.

This is only suggested approach and not a comprehensive solution. The assumptions are that you have limited products sold and managed at reseller and also that updating their inventory isn't very frequent.

Thanks. What is the odoo functionality I can use to input the client's periodic inventory and track it?

Hello, the client's stock can be carried as long as that same client lets you know the sales they have. If Odoo does not have a way to see what your client sells, then it is impossible to carry the stock, but if that client makes periodic inventories and tells you, you can put it in that client's stock.