Questionnaire (research car trivia), then phone screen, then on-site. For the on-site, it's an all-day affair: you meet the operations manager, a team lead, and the finance guy, you do a quick logic test, and then you go for lunch with two more colleagues and wrap up with the operations manager.
The phone screen and questionnaire were straightforward: as long as you can do basic research, have some answers in your arsenal for a fairly standard set of interview questions and know how to build rapport with people, you'll do fine. The on-site is a long day. There were no "bad cop" interviewers - the operations manager in particular is a friendly and relaxed guy who puts you at ease - but you do three hours of interviews (no hard questions, but make sure you've familiarized yourself with how a supply chain operates beforehand) before your logic test, and so although I'm usually exceptional at tests of that nature, I'm pretty sure I crashed out on their math question (after talking up my math skills!) which on any other day would have been a breeze. By the time I had my wrap-up interview with the ops manager, my brain was fried and I had little to say. I didn't receive an offer, and nor was I expecting one. So: more than anything else, get a good night's sleep before you go to this interview, and steel yourself for a thorough test.
One thing: benefits for this position are better if you're young and free of family obligations. (There is a health plan, but a short waiting period before it kicks in, which would have made it near-impossible for me to accept an offer were I given one.) Pay and bonuses are, however, extremely attractive. Also, you will be expected to meet probationary metrics in your first few months of employment (and later, but monitored differently): they seem a little arbitrary at first glance, but RockAuto's alarming growth stats speak for themselves, and everything is clearly explained during the interview.
Finally, this job is not for you if you do not have an engineering or spatial brain: it's really all about spatial thinking.