At the end of each expansion we usually vendor trash any leftover mats to clean the bank. If the beta is an accurate representation of Midnight DON"T SELL OLD MATS. Well at least flowers.
In the new expansion Inscription and Alchemy can make dye for housing items. I thought I would need a bunch of new flowers but turns out I could mill all my leftover stuff from War Within.
Server still lagging bad so no video yet...soon I hope.