On Amazon, T-Shirts With Slogans Expose Frivolous-Trademark Feud

A growing number of sellers on Amazon.com Inc.’s platform are finding how painful it can be when Amazon’s absolute power over their small businesses meets a bureaucratic U.S. government system not built for the 21st century.

The issue involves trademarks filed with the U.S. Patent and Trademark Office, a federal agency meant to ensure brands big and small can protect their intellectual property. The office processes hundreds of thousands of trademark applications a year, and experts at a hearing this month warned Congress that spurious trademarks have overwhelmed the system.

Some sellers on Amazon’s platform say competitors are ...