I wonder whether someone made a living from that, like guys who retrieve and collect golf balls lost in water hazards and roughs, and re-sell them.
It was common practice to salvage weapons and armour after battles so makes sense that it would apply to arrows too. It has happened throughout history and still happens now. Think ISIS who continue to build up their armoury thanks to 'donations' from the field. There surely would have been arms dealers in the 1400s.