A predatory agent is not just aggressive, they are unprofessional and selfish. There's nothing wrong with paying a sizable commission for real services. But if an unproven or unprofessional agent handles your property sale - bad things can happen.
First, you can lose your home. Predatory agents are generally incompetent and unsuccessful at selling, so they target owners of valuable homes, so that if they are fired - they can sue the homeowner. Predatory real estate agents often force elderly homeowners into bankruptcy by suing them and forcing them to mortgage their homes to defend themselves.
Second, you can wind up getting far less when you sell your home. Predatory agents are always rushing to sell as fast as they can, so they often encourage the homeowner to do things that are counterproductive to the goal of obtaining the maximum selling price.
Third, repairs can be shoddy or over budget. When predatory agents are involved in renovation or repairs, they will often hire the cheapest labor, or unproven labor - leaving the homeowner stuck with shoddy work.
Any way you stack it, predatory real estate agents can destroy the premium homeowner, leave them bankrupt, broke and homeless. Some of the red flags to watch are: bullying personality, no track record, rude to guests in your home and sloppy about appointments.
Checkout posts on the Dishonest Real Estate Agents Blog for anonymous testimonials from homeowners who have been victims of predatory real estate agents.