** Updated: sorry I know this isn't specific to the OP but I was just responding to this post about taking advanced payments.
Great thoughts here and I can certainly see exactly what you are saying. Try to understand the below, from a seller's perspective... This can happen if you personally are the seller of a knife so keep that in mind.
Unfortunately, as a service provider I have had to move away from 100% payment upon completion for every order. There is something called 'friendly fraud' out there and it's a big problem for merchants. What this means is that the owner of the knife could have me do work, pay with a card, then once they get the knife back call their card company and initiate a chargeback, stating that the knife wasn't as described (even without contacting me to allow me to fix it or make it right - remember this isn't the goal with friendly fraud, it's to get the cake and eat it too). Their card company immediately seizes the money from the merchant's account and won't give it back until the 'case' has been resolved - guess who decides who's right? The customer's card company. This doesn't leave the merchant with much hope - and no the card company doesn't require that the customer return the item, that's why it's fraud.
So I have had to change my payment policy recently to require 50% down on orders over $400 in value, including making knives - and I require it be in a USPS money order, cash, or a walmart money gram transfer. I try to treat everyone as I would like to be treated, even losing money (as I've done time and time again) to make a customer happy. But these steps prevent (somewhat) friendly fraud from occurring to me in the future as there is nothing they can do after I have the $$ other than contact me. To some extent, it puts me in charge of the $$ where I can call the shots and decide whether or not to refund the $$ (of course I will if I get the knife back). An important point to make though, is that this pre-paid $$ goes in a savings account to stay until the work is completed, at which point I move it over to my regular checking account.