I think that if I were in that business, I would be happy to hold payments for my customers until their names came up on the waiting list. I would go ahead and buy the materials, and still be willing to refund the money if the customer changed his mind before I started the bow, because I could use the material on another bow. Many times I have discussed wood with a bowyer, and have asked him, "What do you have that you think would look good for my bow?" And I have always been pleasantly surprised, because his reputation is on the line when he recommends something. I would not offer financing after I shipped a bow, because that is a whole 'nuther business. Just because someone is an expert bowyer doesn't mean that he is an expert banker (whether bankers are expert bankers is a whole different subject, given their credit losses in recent years!). I would probably not take trades, because the time I spent buying and selling bows would take away from the time I could spend building bows, but some people might enjoy doing that, so that would be an individual decision.
Granted, sending a refundable deposit off to a bowyer is not much different from putting it in a drawer at home, but if it helps some people to save, then why not?