The notion of making things that can't be worn by the current character not show up in the store is really not a good interface.  These items should really show up with a red border or something.  In fact, the character should be allowed to buy them, just not wear them.  Having items disappear just because the current character can't use them is really very confusing.

That said, I think that the store has deeper problems then this.  Some items seem to be available some times and not other beyond the ability of the current character to equip them.  For example I always try to buy traveling boots for each character, and sometimes, even characters that can use them don't see them in the store.  (This might be because they are already wearing a pair?  In which case, the interface is even more confusing.)

I've also noticed that Oak Spears seem to be available in the store sometimes and not others without a meaningful pattern. 

If someone could spend some time looking at this I think it would really improve the player gaming experience.