When you install a pack in YEM downloaded from YMS account, you will be asked for your account information: email and password. If email and password does not fit to your's content pack, the content will not be installed. The pack was paid by previews owner, in its account. You will not be able to download freely that pack (or packs) from your account, even if you registered the same keyboard in your account. I can register a lot of Genos, for example, in my account, if I sell a pack as cpi or cpf (which means I have info file from each user's Genos).
The only solution is like this:
Seller: request Yamaha to delete packs from its account and to transfer to new email address. In the same time, seller has to prove the sale process of its keyboard. Or Yamaha must have a "block" list for keyboard ids, and seller has to unregister its keyboard, and buyer to register the keyboard in its account.
In this condition, the new owner of Genos can download anytime the pack(s) from its account and can install to its YEM installation without any problem.
Regards,
Vali