How about this for a level limitation at "Gnomie the Know-It-All":

Below level 11 = Can lore any weapon ranked below 11
After level 11 = Can lore any weapon with a rank lower then the character's current rank. (Like buying anything else at a store, if you're rank 16 you're not getting rank 31 info/gear/trollop's favors)

This lets characters find the information on any piece of gear they should be able to get easily from it's source.

Limitation possibility #2: Make limited items cost considerably more to identify. Gnomie's way of making sure you're worthy of the info. Say a 2x multiplier in copper price. Possibly some small discount to other Gnomes, as a professional curtesy.

Add in a qualifying question, so that you know how much the lore will run. Interaction runs something like this:
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Lore macethatslimited

Gnomie says, "Ahhh, now there's a weapon you don't see everyday. As this one's special, it'll run ya 2500 coppers for me to tell you about it. Interested?"

say Yes

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Sound fair?

-Falstaff

P.S. Yes, I'm having fun naming the gnome. *chuckle*