Used to work at Carmax. Saw people with mid-500s get approved with Santander/Drive. Like StartingOver10 said, they're the very last fall-through lender that Carmax uses, and their rates are atrocious. When I worked there 4 years ago, their max APR was 23.99%, though I think this may vary by region.
It goes Carmax Financing --> Wells Fargo --> Capital One --> Santander/Drive
For those who were denied even Santander, there was an external company called Roadloans who were a last resort. Even for those who qualified with Santander, Roadloans often gave a better APR. If you're eligible to join a credit union, that's worth exploring too. I explicitly remember that at least once, a customer leaving with Santander financing came back a few days later with single digit financing through their credit union.
One last thing I'll say is that the Carmax sales people work with customers with bad credit all day long. That demo probably makes up like 50% of the clientele with the other 50% being made up of those with average+ credit. Despite that, I never once heard any sales people talk badly about or make fun of any customers' credit. As long as you're realistic and practical about your situation (eg, not trying to get into a $35K Mercedes with your $30K income and $1K down payment), they'll treat you with respect and do their hardest to get you into a ride. They get paid EXACTLY the same whether they get you into an $8K Hyundai Accent or an $80K Porsche 911 Turbo (literally), so all customers are treated the same.
