Identifying your target market is key. Your target market is the group of people who will buy your cards. Getting the word out about your product is your marketing plan. Knowing your target market will help you define your prices and determine where to market your cards. For example, if you sell cards which will appeal to hipsters, your target market is going to be very different than if you are selling cards for a kid's birthday.
My granddaughter starts kindergarten this year and she has sight words she'll need to know. We made her a set of flash cards with all 300 of her sight words for the year. I needed to get them done and shipped off so she could get a head start and they were here before I knew it. Easy to use, did a mail merge and printed them all out with minor corrections, mostly of the mea culpa variety.
Keep in mind that there are already many handmade greetings cards available. To be really successful, you need to offer something a little different. In many areas, the market is swamped with handmade cards. An example of a niche is a "green" handmade card business where all products used are recycled, sustainably sourced, or some portion of proceeds benefit ecology projects around the world. When you are thinking about a niche, a great place to start is to think about the things that inspire you or make you smile.
Making and selling handmade greeting cards provides the opportunity to simply earn a few extra dollars or to build a significant income. Most small card businesses fall halfway between these two extremes. Handmade cards remain popular and many people like to give handmade cards to mark birthdays and other special occasions. The greeting cards market is a $7.5 billion annual industry.