Before starting a handmade card business that uses artwork from another designer, be it in a stamp form or ready printed, check whether this artwork comes with a commercial use license or if you have to pay for the licensed image (once or every time it's used)? If you do have to pay for the image, make sure that cost gets factored into your pricing formula as you determine the per-unit cost of your cards.
The presentation of the cards—the packaging—helps make a successful handmade card business. A card that is neatly presented in a cellophane envelope looks more professional and can command a higher price than one that is unpackaged. Presenting cards in some form of packaging stops cards from becoming dirty or dog-eared and it also gives the ideal opportunity for further marketing. A label on the back with your phone number or website address could help you solicit further orders. Remember to consider shipping and packaging when factoring the costs per unit in your pricing formula.