Qualifications
You'll need to check your individual state's licensing and bonding requirements, as they vary by state. Most states require repossession agents to have special insurance and be bonded. If you're planning on going it alone as an independent contractor, you'll also have to have some money for buying a tow-truck and paying for other incidentals.
Other than that, with a hefty amount of gall, common sense, and street smarts, you can do this job. It helps if you know how to hot wire an engine.