Amazon Account with Purchases in Excess of 50$
Posting Reviews on amazon.com or commenting on them is only allowed for Amazon accounts that have made purchases with a total value of at least 50$.
Seller Central User Permissions are not relevant in this regard.
For Vendors: The account may also have to be registered in Vendor Central.
Steps Taken against Fake-Users
This is one of many new steps taken by Amazon to counter the large number of fake reviews and comments. It is not unprobable that these requirements might become even stricter in the future.
Use Private Account with Company Name
Typically sellers will use a private account with which they have made enough purchases and give that account a professionally sounding user name to appear on the comments, eg. "Company Customer Service".