first thing is that non-inventory is a misleading name, it is really an expense item. It does not hold qty or value, so when you use one in a purchase the amount is sent to the expense account on the non inventory item screen immediately

Answering your question is a harder.

If you have expensed the items in the past, then selling them is income in the present period

If the items were listed as assets when you purchased them, then the sale is income and you expense the asset value.  If they were listed as fixed assets and depreciation was taken in years past, that depreciation also has to be used to lower the expensed asset value.

Since there are so many if this then that possibilities that is the best I can do for you.