A concern has emerged that junior and senior high school students will experience troubles in using flea market apps due to their spread for conducting inter-personal commerce through the Internet. This study thus developed a lesson focused on the troubles in using flea market apps and practiced it for second-grade students in junior high school. The results of post-questionnaires,multiple-choice tests,and free descriptions demonstrated that many learners understood the precautions related to using the flea market apps and considered their use based on such understanding. Further,this study found room for improvement in the way of explaining certain contents.