You can sell A Siyum OFF EBAY and make any terms you want, If you own a Judaica Store on 13th Ave, but when you sell on their platform you have to obey their rules, it is a secular platform, so you have to follow secular rules. They do not have to let you sell on their site if they dont want you too.
I know people who got kicked off for various reasons and tried to sue and they were not able to win