What sells on Zazzle? Well, I don’t have the stats—Zazzle folks do. 🙂 I did notice that the types of products that are popular in my store are those with [cup]cakes on them (e.g., http://www.zazzle.com/birthday_cake_pattern_on_blue_binder-127384430078442986). Other stuff gets noticed too, but cupcakes rule!
Looking at other sellers, especially pro-sellers, I see that some cute designs that are popular (am not into grunge or politics or dirty humor) and/or the kind of designs that are bountiful at pro-sellers’ stores are those related to various celebratory events. In particular, weddings, baby-anything, graduations, and of course holidays, but mainly—bridal & baby stuff. Cupcakes/muffins/flowers/pastels/birds/rich backgrounds/invitation forms/envelopes/notes/cards seem to be the most-sought-after items/themes among customers.
These can be interesting to design for sure, and I have created some and will do some more, but only as long as it is fun. Otherwise, it would feel like a commercial enterprise. Ten hours a day designing/marketing bridal-themed items, like some pro-sellers do—no, thank you. Personally, I only do this for fun. If it sells—great!
On another note—I’ve seen many stores where it is clear that the owner is very skilled at drawing and/or at designing using illustration software, but they are not a pro-seller. So, being a good artist by itself is not the key to being successful at selling products (although it can’t hurt, obviously).