There are a few things that you need to do though to make sure that we can accept your content legally.Please make sure to include a license with your work. Any content that you want in Sandbox itself must be able to be used commercially. If you need help deciding what license to put your work under check here for help:
http://creativecommons.org/choose/
If you submit content that includes work done by other people you MUST make sure to include the license of that work and make sure it can be used commercially if you want it in Sandbox itself.
Thank you again for helping Sandbox! Take care
-mike