Hi Mikaela, 

Welcome to the Box Community, I'm happy to help!

Looks like eBay's platform does not permit the use of shared link URLs, so you will have to upload the actual file on their website. You can download the image from Box into your computer then upload it to eBay, also it seems they only support  JPG, GIF, PNG, TIF or BMP files so you want to make sure you only upload any of these supported files types.

Thanks for posting and let us know how else we can help!

All the Best,