Listen for a frequent "buzzing" or clicking sound from under the hood when the car is parked or the brakes are applied. If the ABS or Brake lights are on, walk away—this repair is often $2,000+.
What is your or the specific year you are looking at so I can give you a more targeted checklist? how to buy a used prius
Each Prius generation has its own personality—and potential pitfalls: Listen for a frequent "buzzing" or clicking sound