1. Enable captcha by adding captcha keys in Joomla captcha plugin.
2. Set default captcha in Joomla global configuration.
3. Check seller registration and customer registration in frontend. They should require a captcha validation before registration is processed.
I configured the menu for registration selecting the page: Sellacious - Register, but the Recaptcha doesn't appear. However, I configured the same menu but selecting the conventional page: Users - Register (the normal page) and it worked. So, the Registration of Sellacious doesn't work but the Joomla registration so works.