Do I fill out "disposing of non depreciating asset if I sent my Mary Kay Inventory back for a repurchase of 90% in 2020 ?

You generally need to report this as a cost of goods sold in the year that you disposed of your business. You will need to amend your 2019 return to reflect your cost of goods sold even though you did not dispose of your inventory in 2020. 

If you deducted the total cost of the inventory as an expense on the 2019 return and don't have it listed in the inventory section then you must claim the buy back as income on the 2020 return.  

I think we need to understand a little more about how you maintained your books and records before answering your question:

• I assume you are a sole proprietor filing a Schedule C
• What method of accounting did you elect?
• How did you treat the inventory when you purchased it?