The Drunken Lizard Pub

What are they used for?

Nothing! Just one of the items you can find. Sometimes you find junk.

but why can't i sell them, then? the vendors are telling me to keep them.
i thought about something like a hidden uber weapon.
maybe if you keep the holy metal, the mithril and other stuff, get enough weapon fragments? there may be some way to craft something or get it crafted.

No self-respecting shop owner wants to deal in junk. Just like the widget, no real use for weapon fragments.