If you want to report someone who sells illegal diet pills, you can try the following methods:

1. To report to the local market supervision department, you can call the report hotline 123 15 or 12345.

2. Report through Internet reporting platforms, such as 123 15 Internet reporting platform and State Administration of Markets.

3. Report to relevant government departments or institutions, such as consumer protection organizations and trade associations.

4. If you report through the report mailbox, you can find the report mailbox on the official website of the relevant government department or institution.

It should be noted that the reporting behavior needs to be true and objective, and it is not allowed to fabricate facts or falsely accuse others. At the same time, the content of the report needs to be as detailed as possible, including name, contact information and report content.