Can I use someone else design?

Due to copyrights laws, you can't use someone else design if you do not have any right to use it.

You must ensure that you either own the copyright in any Merchandiser Designs that you upload or submit or that you are fully licensed by the copyright owner to use the Merchandiser Designs to personalise products and to sell the Artwork in the form of Merchandise Products, that you are not infringing any third party's trademark, or breaching a duty of confidentiality to any third party.