Select and Display Products Not Sold

SELECT DISTINCT
    i.description, p.invID
FROM 
    Inventory i
FULL OUTER JOIN 
    Purchasing p ON I.iD = p.invID
--WHERE i.invID

Returns:

Socks       1
Ties        NULL
Underwear   NULL

With where unblock this T-SQL should show Ties, Underwear but shows nothing.

Where am I going wrong?