For iron arrows (and possibly some other low value arrows), you will not get anything for them at the start of the game. You're better off not picking them up unless you intend to use them, if you don't like them cluttering up your inventory. Or store them somewhere if you can until you see that you can make get some money for them. OTOH they don't weigh anything so it's not like carrying them around is a problem.
It will take a higher speech skill and/or some Haggling perks (or potions or enchanted gear) before you will be able to sell them rather than give them away. It's always a landmark moment in my characters' lives when they hit up a merchant and find that he/she is suddenly willing to pay 1 septim each for iron arrows.
Edit - I think on my current character I had to take the first two Haggling perks plus the one that gets you better prices from members of the opposite six before I could make anything selling iron arrows. Can't remember what my speech skill was but probably in the medium range since I do a lot of bartering and tend to level up fairly quickly in that skill.