Law enforcement is just that -- enforcement after the crime has been committed. It cannot protect you from a crime that someone is thinking about until they actually begin doing something illegal.
Telling someone you will sell them your $15,000 car for $1000 is not a crime, even if you are lying through your teeth. Taking their money and not delivering the goods is. Emailing back and forth about a supposed sale is not a crime until funds are exchanged and the goods do not change hands. Law enforcement cannot do anything until after you have suffered a loss, unless the perpetrator can be shown to have victimized someone else.
If you want to be protected from crime happening to you, you need to plan to do it yourself by whatever means you are comfortable with. If you want someone punished after they have harmed you in some way, call the police. You can be pretty sure they will not be able to arrest someone who is thinking about breaking in to your home, but they might be willing to look for them after the fact.
You need to accept the fact that the law is simply a set of guidelines more or less defining which actions will result in attempts at governmental punishment. Protection of person and property from being victimized is up to the individual, with the law being relegated to attempting capture and punishment.