Think of it this way -- you did your best to sell him something which fits him right and has the potential to kill a deer. At least he got that much. You can't sell brains, and you aren't responsible for what he does with it. If the situation presents itself again, do your best to be helpful and knowledgable. Talk to your manager and see if they would let you put up a buliten board with archery clubs in the area and people who give lessons. Maybe then, you could use it as a conversation piece. Something like, "Be sure to check out our board! Threre are archery clubs all around who offer beginner clinics if you are interested!"

Who knows. Maybe you can help someone.