Okay, I looked it up b/c it was driving me crazy. Here is a copy of K12's terms. Maybe you could look up the terms of the materials you are selling. I'd be very surprised if most HS companies have a problem with people reselling, as they know that is how a lot of HSers get by. Anyway, to satisfy my curiousity and maybe yours:
"Can I resell my printed materials?
No. The Student and Teacher Pages by K12 Inc. are intended solely for
the use of the family who originally purchased those products; and
the customer is not allowed to resell or transfer the intellectual
property in those products to a student who is not a part of that
customer's account."
And the Terms of Use from the OLS (this part is an online thing):
"PROPRIETARY RIGHTS
You acknowledge and agree that all content, software, and materials
available on the OLS are protected by copyrights, trademarks, service
marks, patents, trade secrets, or other proprietary rights and laws.
Except as expressly authorized by K12, you agree not to sell,
license, rent, modify, distribute, copy, reproduce, transmit,
publicly display, publicly perform, publish, adapt, edit or create
derivative works from such materials or content. Notwithstanding the
above, you may print or download one copy of the materials or content
on the OLS on a single computer for your personal, non-commercial
use, provided you keep intact all copyright and other proprietary
notices. Systematic retrial of data or other content from the OLS to
create or compile, directly or indirectly, a collection, compilation,
database or directory without permission from K12 is prohibited. In
addition, use of the content or materials for any purpose not
expressly permitted in these Terms is prohibited. The K12® Online
School is patent pending."