I have a shop on Zazzle but it does not perform well. But outside the Redbubble, and Etsy, I have best experience with Spreadshirt. The upload process is easy and they have one huge advantage. Spreadshirt has in fact two marketplaces. One in US, and one in UK for European audience. While you need to sign up for both, and upload the same design there, the competition on the UK marketplace is low. Many sellers do even not know that it exists.