I'm also experiencing this and it's driving me insane. Oh, you wanted a greatsword? Here's an Adamantine greatsword I just made yo- aand you've decided to take a Mithril spear off the shelf, and pay me for a Mithril greatsword. I just wasted money.
Edit:
SOMEONE JUST CAME IN ASKING FOR A POLEARM
I HAD A MITHRIL POLEARM SITTING ON THE DESK
SHE TOOK A TITANIUM SWORD OFF THE SHELF AND PAID ME $17