we had a similar problem regarding the value of our trade at round rock. they were only willing to offer me $5500 for my '95 blazer, and offered a kelley blue book web printout as support. having done my research, i was amazed at the number. i later went to the kelley website to see for myself what was up. it turns out that in order to arrive at the $5500 quoted value, round rock not only listed my truck as being in "fair" condition (something i had disputed immediately), but they also stripped every available feature from the value calculation equation. that is to say, they failed to enter the appropriate add-ons, AND WORSE, they actually removed all of the options that automatically pop up as "standard" equipment (such as a/c, power steering, AM/FM cassette, etc.).
so i ended up working w/ south point as well, and was generally pleased. be careful!