Why not?
not a problem as long as he makes his commercial interests clear before making any ‘influencing’ statements on public forums or publishing reviews in the public domain.
About users with hidden agendas
If you know that a user is a dealer and they fail to disclose and attacks other users because they make a point against their interest, do you expose the user?
I know of a gentleman here that continuously posts and goes after people and does not disclose who he is.
What are the ethics of this forum?