The website is easy, or you can go to places like Shopify and make a shop.
Then establish your point of difference from Hallmark etc.
The hard part that will consume 80% of your working time, is marketing.
The whole business succeeds or fails on your marketing skill, NOT your site or your designs. Finding niches who want your product and marketing to them accordingly. E.G. Greeting cards for pets, or vets to send to pets as part of THEIR marketing.
Answer by Graham Campbell: