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