This is only part of the issue ("Guarantees").
The far larger and more important issue is disclosure laws.
As the other guy posted, you cant recommend one thing and do another.
"we recommend you BUY XYZ" while you are holding a lot of shares in this and sell into the buy market you created.
You also cant offer recommendations without disclosing how you are compensated.
I'm Canadian and more familiar with its rules / regulations but Canadian rules are often best practices used globally.
https://www.iiroc.ca/news-and-publications/notices-and-guida...
Disclosure of financial interest (clause 3608(2)(ii))
A Dealer must disclose whether any person involved in creating the content of a research report has an ownership interest in the subject issuer’s securities. When disclosing ownership interests, a Dealer is not required to include information relating to administrative or clerical staff involved in preparing a research report.
Disclosure of remunerated services (clause 3608(2)(iii))
Clause 3608(2)(iii):
does not require duplicate disclosure from the individuals when the Dealer discloses the services, and
excludes normal investment advisory or trade execution services, such as an investment account by the issuer.
Disclosure if making a market (clause 3608(2)(vi))
A Dealer must disclose if it is making a market in an equity or equity related security of the issuer. In addition, a Dealer must make the same disclosure in a fixed income research report if the Dealer is making a market in an equity or equity related security of the issuer.