Hi Ankit,
Go to YOURSITE/sellacious/index.php?option=com_sellacious&view=emailtemplates and click on email template for SELLER REGISTER => USER .
Set the option "Sent to Actual Recipient" to YES . If it is set to NO then you must provide valid email(s) either in Recipients, CC or BCC fields.
As we have tested out the seller registration on our demo site and it is working fine.
if your issue still persists then you can send us you site url and credentials via direct message so that we can look into it.
Could not instantiate mail function.
it could be due to your server error as mail setting are not correctly set.