Hi, Mr. Bouchard. I'm Eskobar, a moderator for the SCP Foundation wiki and a member of the Licensing Team. We noticed that you had up for sale a sweatshirt design depicting the SCP Foundation logo and the "Secure, Contain, Protect" motto.
As a member of the Licensing Team for the wiki, it's my job to inform you that SCP-related media or depictions used for commercial purposes are fine from a legal standpoint, but that three specific requirements apply:
1) Due to the nature of the Creative Commons Share-Alike 3.0 license that the SCP Foundation wiki and all related works are released under (link here), all derivative works are required to link to the source from which you derived the work. In this case, you can include in your description a link to the SCP Foundation wiki as a whole at http://www.scp-wiki.net.
2. Also due to the nature of the Creative Commons Share-Alike 3.0 license, all derivative works must be licensed in the same manner as the work it was derived from. In this case, your derivative work must be indicated to be licensed as CC-BY-SA 3.0 (details included in the license link) or a compatible license.
3. We personally want to make sure you understand that the SCP Foundation wiki does not, at this time, endorse or support any merchandising effort. We have no authority to regulate or prevent any such efforts, but nor are any such efforts to be considered or treated as "official" in any way.
Thank you for your cooperation,
Eskobar
Moderator, SCP Foundation Wiki