When you are selling digital downloads on Etsy, it is important to protect your work so people cannot steal or copy it without permission. Watermarking is a common way to protect your images and listings in general. You could put a tiny logo or other identification on the file to deter some content stealers, but they can be erased, so there is no assurance. You can also flatten the design so no one can edit it and claim it as his own. You can read about this here. I usually do this for my printables, it is not 100% foolproof, but it does the job. Including your licensing terms with your download is also a good idea.  You should also mention the usage of your products in your shop policies. Remember to keep your files safe and backed up! There is nothing worse than losing all your hard work because of a computer crash or some other mishap.