I lamented this same frustration in another thread.
The crystal limitation is completely unnecessary and nonsensical. If you are going to retain such an arbitrary barrier, then at least let Eveline ALSO exchange for crystals, somehow (by whatever nonsense ratio satisfies).
Regardless, Northelm still needs to be increased and/or only the name of the item should count towards the inventory limit and you can have as many dupes or variants as you want.
Either way the current system is a nightmare.