How do I purchase arrows from the fletcher? the UI offers "0 units for 0" with left-right arrows, but the left and right keys do not change the offer.
You use the left and right arrow keys to increase the amount you are going to buy. If you can't go above zero, it's probably because either you have no gold to pay for them OR you are already at your maximum carrying capacity for arrows (24).
(Let me know if neither is the case and you're still unable to change the number of arrows in the UI, because then that would be a bug!)