This is just anecdata. We bought a used car at Carmax and they gave us way, way, way above bluebook value for our trade-in (and all other offers, too -- and we were pretty aggressive in researching trade-in offers at other places and were prepared to be underballed by Carmax).

My anecdata is no more useful than yours. Unless someone provides some actual reliable data points to produce a real, known trend, there's no proof here.