When uploading your artwork to create your products you do not need to use crop marks within your files.
Here at MOO we print using a bleed and safe area which does not require crop marks, instead our bleed and safe area specifications will help us to identify where to trim your product(s). If your design files contain crop marks please make sure to remove them before uploading to our site or these could print on your final product(s). You can find our product templates and design specifications here.