What is the name of your state? Washington what is considered a control buy? if a dea agent came in with a informant to buy drugs and the deal did not go through but the agent seen the drugs is this a controlled buy? is this enough probable cause to conduct survelince?
The police do not have to show probable cause to conduct surveillance. However a DEA agent personally observing a controlled substance in the possession of someone who they have reason to believe is buying, transporting, possessing and of course selling narcotics, will most likely guarantee further investigation of the suspect, his friends, family and acquaintances.
Also, a search warrant can be applied for and granted to law enforcement agencies without a DEA agent, or any other detective/narcotics investigator being present during the actual drug sales transaction. In fact, a sale, or controlled buy doesn't even have to take place. There are many other ways to justify their belief that someone is involved in the trafficking of controlled substances.