The Supreme Court and virutually every court have upheld the use of confidential informants to apprehend persons accused of dealing drugs.  You can raise an entrapment defense, but you do have to show that you would not have engaged in this type of activity, but for, the police's involvement.  Once you do this, you could open the door to other transactions coming in (that you may or may not be aware of).  Before you go this route, make sure that you visit with a criminal defense attorney-- because entrapment frequently backfires as a defense.