I am a bit confused, during my first quest I am looting many items, but I didn't find a way to know at which price an item can be sold later. Is this a design decision or am I missing some info somewhere?
There is no way to view how much gold and item is worth outside of a shop, as far as I have seen.
This is probably because the gold value of an item doesn't matter in Torchlight 2: if you can't find a use for an item then give it to your pet and send it off for sale. This means you should be picking everything up, and using only the best of items for yourself while selling off others.
However, reward items from quests do display their gold value before you choose one, so if you plan on using none of them you can pick the highest gold value item.