I feel that the thing people don't like about Residium is that they envision it as an object all its own. Some sort of fairy dust you scrape off. What if it was instead a term for a class of objects. Jewels, Pheonix Feathers, Basalisk Venom, and yes, Fairy dust.. that was used in the creation of magic items. By taking that stuff back you can use those rare components in the creation of new magic items and thus save yourself some of the cost of a new item. Otherwise you'd have to buy some more eye of newt and that can get expensive.