In general, the creation of multiple user accounts to self-promote is prohibited.
In paritcular, the creation of multiple user accounts for the primary purpose of covertly promoting the sale of AAS is categorically prohibited.
This obviously includes sources who including multiple (fake) accounts to self-promote and/or attack unsatisfied customers, critics or competitors.
However, there are instances where I have always strongly encourage members to create an alternate account. They should ALWAYS use a secondary account when discussing interactions with a source, especially negative interactions and experiences.
Sources are known to retaliate against members who post reviews or criticism that could adversely affect or threaten their business. The sources have your name and address. So it could be dangerous for members to publicly criticize sources. A secondary account is a reasonable precaution.
Other situations where multiple accounts may be permitted is when a member decides to source. The primary account could likely contain personally-identifying data given the amount information people tend to share as part of a community. There is expectation to force anyone to self-incriminate.
Other situations where multiple accounts are prohibited apply to banned members. Banned members are prohibited from circumventing their ban with additional user accounts.