First step is the online assessment which measures both your sales and customer service aptitude. If you pass this you will receive a short phone interview. Next process is done in store, plan for 3 hours. First you must verify your answers from your online application and explain any differences. The next step is an interview regarding your customer service skills, typical questions are "Tell me about a time you went above and beyond for a customer" or "What does excellent customer service mean to you?". The next step is a sales based interview. You will be asked to sell something I was asked to sell the interviewer my pen, be sure to ask probing questions and point out features and their benefits. If you pass these interviews the next step is an interview with the General Manager at which point you have a 99% chance of being job offered. They will explain the pay structure and ask you to take a drug test. Just for reference they have you do it right then and there and UPS it to the testing center. If you pass the drug test and background check welcome to CarMax.
You will generally have a new Manager at every point of the interview process and if any of them don't feel like you'd be a great fit then you are out. Working there for two years I many people that didn't even make it past the first interview. Dress nicer than you think you should and eat your Wheaties because you will need to bring your "A" game. If you don't get an offer the day you interview then you didn't get the job.