first thing under photos, in caps, write "This is a custom item. Will take 3-4 weeks to make before shipping"
then reiterate, a few different ways a few different times. Also writing custom or made to order in the title (again, in all caps) helps. it sucks to have to be so obvious about it, but sometimes its needed.
In all our listings, we also state if a listing is for arrows, just a bow, a bow with arrows, etc. It helps keep everything straight when someone gets upset for misreading.