There are many ways to pick which products to place your products on, and there are several things to consider.
1st do you want your store to be an artist gallery that happens to have the items for sale, or do you want your store to be a retail outlet that just happens to sell your art?
In order to make this decision there is a lot to consider.
Option 1 - an artistic gallery which happens to be for sale.
In this case you would individually add your items, only adding the design on the product which you feel best represent the artistic value of your art. This may not lead to the most sales, since you may not offer the product wants, but you will maintain your artistic integrity which could be lost when you see your work on a product you do not feel it should be on.
Option 2 - a business that sells products which happen to be artistic
In this case you could create a template section in your zazzle store including one of every product with your design placed appropriately on it. Then you would be able to use Quick Create to add each design you have on all those products. This will result in some products you may not find perfect. People with this type of shop live by the mantra "I am not the shopper, but I will give them what they want, even if I would not buy it myself."
This may lead you to sales of unexpected items, such as adult themed designs on children's sized shirts, or women's themed designs on mens tshirts. Remember you are not the shopper, and you do not know who that item is being purchased for or by.
Several years ago I found myself quite shocked to see an adult themed design of mine sell on a child sized shirt, until i received an email a few hours later thanking me for offering all my designs on all sizes of products. The purchaser then continued to write informing me that he often had difficulty as a little person purchasing age appropriate clothing in the "children's sizes" which fit him correctly.
As for womens designs on mens clothing, many women prefer to buy mens cut shirts, or need larger sizes which are often only provided in mens cut shirts. I have even met shopkeepers on Zazzle who are women, who prefer men's cut clothing who only offer their clothes on women's cut shirts. Even though they as shoppers would prefer to buy them on men's cuts! keep this in mind when considering products, the audience you do reach, may be different from the one you intend to reach!
for more on how to make a template and use Quick Create check out My Tutorials
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