Write an article based on this summary:

Learn about types of mail fraud. Gather information about the transaction. Contact the Postal Inspection Service. Complete your mail fraud complaint. Cooperate with any further investigation.

Article:
If the auction or sale proceeded to the point that the U.S. postal service was involved, the perpetrator could be guilty of mail fraud. For example, a dishonest seller may have pocketed your money and not sent the item you purchased, or used fake user accounts to bid on their own merchandise and drive up the price. If you mailed a payment and received no product in return, or if you received something in the mail that didn't match the description on the listing, the activity may constitute mail fraud. Understanding the elements of mail fraud enables you to choose important and relevant information to include in your report. You should focus in particular on the involvement of the U.S. Postal Service. If the postal service was not involved in the transaction, mail fraud probably did not occur. However, if you paid for shipping and did not receive the product, the postal service is involved – even if indirectly. The Postal Inspection Service investigates reports of mail fraud from individuals.  If you want to report mail fraud, you can contact the Postal Inspection Service by calling 1-877-876-2455. Option "4" allows you to report suspected mail fraud.  You also can mail a letter or report to Criminal Investigations Service Center, ATTN: Mail Fraud, 222 S Riverside PLZ STE 1250, Chicago, IL 60606-6100. You can submit a written complaint online, or draft a letter with a description of the incident and relevant information.  The Postal Inspection Service provides a complaint form at https://ehome.uspis.gov/fcsexternal/default.aspx that you can fill out and submit. You must include your name and contact information – you cannot submit a mail fraud complaint anonymously. Once your report is received, a postal inspector may contact you for additional information. Since you cannot attach any documents to your online complaint, you should retain any receipts or documentation that may provide evidence for postal inspectors investigating your complaint.