Location, location, location...
I've moved around the country quite a bit, and found buying/selling habits to be quite different from place to place.
For instance, the above poster with the $2600 1990 CBR1000F would have no problem selling it at that price in my present location (Richmond, VA) as liter bikes sell for a premium around here.
There are factors that others have already touched on, like the almost-new/new decision a buyer makes.
Your previous sale of a VX800 is an interesting comparion. That bike was fairly limited in production, and has a semi-fanatical base of followers (almost like the old Honda Hawk). It was no surprise to me that it sold quickly.
Your ZX-6E, however, despite outperforming the VX800 in all performance regimes, is pretty dang ubiquitous, Kawasaki having produced it, unchanged, for several years now. American riding habits being what they are, a buyer could expect to buy an older bike with very low mileage, for a substantial discount over yours, with a mere year number to differentiate.
I generally deal at the lower end of the pond, and I can tell you that buyers' appetites for decent running bikes in the $1500 remains strong.
BTW, it ain't the economy. Haven't you heard the news? The recession ended 2 years ago!
Nyuk, nyuk,nyuk!