We should probably also sell spirit dusts, draco shields, and talismans. I feel the only stat boosters we should be buying/keeping from an efficiency perspective are energy drinks, seraph robes, and speedwings. And energy drinks might be better off sold for forges/arena/whatever.
The spirit dusts have the problem that sages can only hit 25 speed max so we probably shouldn't bother using them. I know you might want them for etzel after reading the argument last page so it's the tier players call.
If I sell 2 dracoshields or talismans and buy a seraph robe with the money I'm actually better off than if I had used them since I get both a +3 larger boost and the boost applies not just to enemies who attack defense but enemies who attack res as well. Same cost better results.