People buying crafting materials either need tons of them for leveling or a handful for a specific recipe. There's much more money in selling one-by-one, but that also involves much more labor. Basically the question you need to ask yourself is whether or not it's worth your time to post items one-by-one and repost frequently, or whether you're better off just sticking a stack of 99 up, waiting longer for it to sell and earning less per item.