Dear,
Is there any feature that buyers can also log in and feed the purchase details in erpnext. After that, we will receive it through the notification and can contact with the buyer and make Sales Invoice and fulfill the requirement.
Make a new user role and assign Doc permissions according to need. Workflow can be used for notifications.
I have created a user. now which type of user permission and role permission i have to assign to user that they can easily request for material through it.
And how to convert it into website user.
It would be Customer. You would need to tweak the profile a bit to give appropriate access.