Go where your final customers congregate in large numbers. This may be a convention, trade show, or flea market. Display, demonstrate, and get feedback about what you have to offer. Pre-payment is a way of getting your customers to finance you. If your product or service is really desirable you should have no trouble especially if you add a discount for cash up front.
Pre- payment is customary in mail order retail businesses and at home sales parties.
Changes of heart and refused C.O.D.'s can nip a budding business before it gets a chance to blossom. Be concerned only with the image you present to the public. Your workshop can be a disaster and your mail room a dungeon. If your work is quality and your product does what you claim it will, no one need ever know. Put your money into your sales image.