Tricky Business — Franklin W. Dixon

IN A TRICKY BUSINESS THE BOTTOM LINE IS DANGER!
A business organization calling itself Trusty Teens depends on a young sales force to peddle its products from door to door. But while there are plenty of teens to do the dirty work, trust is short in supply. When a mysterious explosion rocks the warehouse and young Jimmy Quayle is accused of attempted murder, Frank and Joe decide to set up a sting of their own.

The Hardy boys infiltrate the main offices of Trusty Teens and uncover a secret sideline. Frank and Joe must shut the business down -- before the whole operation goes up in smoke!