The use of audio equipment for covert listening or mechanical recording or
interception has been discussed and the law is fairly universal in the US that
such activity is prohibited unless at least one participate to the conversation
consents to the listening by a non party to the conversation.
     Video surveillance has not had the same legislative coverage, and the use
of "nanny" cameras is something a lot of alarm companies are interested in, and
many if not most sell, install and service CCTV.
     There is a lot of proposed legislation to regulate video surveillance.
You need to check to see if your state has enacted it, or whether it is
proposed.  You need to make sure you are not committing a crime by selling or
installing the video equipment.

     Here are a bunch of proposed bills that are pending in New York State:

Schimminger (MS) -- Provides for the creation of the crime of surreptitious
video surveillance
Brodsky (MS) -- Creates crime of unlawful use of surveillance in a public
place, without posting notice, prescribes civil and criminal penalties
Tocci  -- Prohibits unlawful observation or video surveillance in certain rooms
on premises used for commercial purposes or which provide services to members
of the public
Oaks (MS) -- Provides for the creation of the crime of surreptitious video
surveillance
Rules (Levy) -- Relates to the use of video surveillance equipment in public
areas
MARCELLINO  -- Provides for the creation of the crime of surreptitious video
surveillance