I’m almost embarrassed to make a whole thread about this, but I refuse to look at an FAQ for fear of spoiling something, so here it is: Can I just immediately sell everything marked as “barter item” or is there some other use? (E.g. in the original Xenogears there was a location that paid extra for them later in the game.)
I personally found no such location. The same thought crossed my mind too.
Yes, sell them off right away. The only exception are the Sephirotic Canes which net you Ziggy’s ultimate weapon once you get 99, but you won’t get any of those until much later.
As long as this thread is here, any tips on beating ΩID? He keeps killing me with double Demon Lord every time I get close to finishing him off. By the way, while I thought that the nods towards Xenogears were getting a bit out of hand (sup, Deus), I kinda giggled when I realized that Universitas>Universe/Space>Weltall 
EDIT: Nevermind, I beat him.