Well, after having filled my Etsy shop with a decent buffer of sketch cards six months ago, I'm finally seeing a jump in sales.
While paintings are my focal point on Etsy, I also like to do sketch cards and larger illustrations simply because they have a quicker creation time. I'm also told that the larger store you have, the more eyes you get browsing your shop.
As an added bonus for fun, I like to doodle on the envelope of the sketch cards I send off. For example, I sent this 1957 Fly sketch card with a mock movie poster drawn on the envelope. I think it makes the experience a little more exciting and personalized, I hope my customers do too.