Hello,
Some of your product are using the "group after purchase" feature ; so it require that the user is a regular Joomla user.
Otherwise, the feature/plugin won't be able to do his job ; it can't change the user group to a guest account because there is no user.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.