Their offer is based on comparables with adjustments for options and mileage.  When you look at the adjustments, most features that cost thousands and value in hundreds when comparing used (options are over priced.)  Find the comparables and see what the dealers will sell them for.  It is fun when you finds SFs comparable and the dealer does not negotiate so get a quote from the dealer that won't negotiate, come up with another half dozen comparables in you favor and they will often throw out the outliers and adjust value.  When I totated our car in a deer hit I was able to find all 7 comparables that SF had used and got quotes from all of them and all but one matched SF for straight out no trade price and I never provided SF price to the dealer.  IIRC it was SF was at 92% of asking