Tony Zipparro breaks down CarMax's (KMX) earnings beat driven by 14% higher car sales and a 43-cent EPS surprise, but says the real story is the financing side. CarMax Auto Finance generates about 60% of profits from just 5% of revenue, with subprime lending growing from 10% to 22% of the business. He warns that rising rates and a weakening economy could squeeze lower-end borrowers, and is taking a wait-and-see approach technically around the $60-65 level.