My DS (24) used to work for them. They give a fair price based on several factors. First, they do not use Kelly Blue Book. It's not that accurate anyway. Second, if a car has high mileage or is over 10 years old it will be bought as a wholesale car and auctioned off. They also check cars pretty carefully for clamp marks and turned bolt marks both of which indicate prior frame damage that has been repaired. That lowers a cars value. They are successful for a reason. Before my DS worked there we sold my 9 yr old minivan to them for $1800 which was $800 more than the dealer was going to give us in trade.
BTW, most cars that are under 10 yrs old, have no body or frame damage and aren't high mileage go on their lot for sale. The rest go to auction.