I doubt anyone would buy ecards since there are many sites offering them for free.
As for real greeting cards, you would design them and print them. That's the easy part probably.
But you would have to have a marketing plan. That's the tough part. You would have to go to managers of stores and convince them to try out your cards. You might want to expect some rejections since they already have lots of cards. BUT if you have something UNIQUE about your cards and a good marketing plan then you can do it.
Another way is direct selling. That way you can sell them without having to go thru the big retailer hassle. You can also have affilliates. Where other ppl sell the cards for you at get a commission of all their sales.
You can go to local businesses and ask them to set up a display of your cards. In exchange they can have a percentage. You can sell them yourself through home parties and such. You can print out catalogs and pass them out. Look at the costs of that.
You can also make your own website (easy) and offer cards for sale through your site. You would just need a domain name, hosting account, and make a simple website. You can use paypal to accept payments which is free. If you need help setting up a hosting acct or domain name lemme know.