my friend sold his e39 540i to carmax. it was in excellent condition (i made sure of it and polished and maintained the paint interior, under hood etc for him). it also had low miles about 38K miles.

carmax basically gave him good condition price (about 800 short) for a car that was in excellent condition. carmax is also a car dealer. don't expect miracles. if you're upside down on your loan... good luck.