You can legally sell any items you own the copyright to. That includes any materials you create yourself using non-copyrighted images and fonts. There are tons of free resources and design materials that don’t infringe on anyone’s copyright. But if you want anything extra special — like a specific font or design — you can buy licenses on platforms like Creative Market. Do not use commercial fonts or images that you haven’t licensed. You risk a lawsuit, and with so many free resources available, it’s not worth it!