I'm in the market for a used English saddle as my parents have now ALLOWED me to have one! So, I need to learn my stuff, and start shopping!
These three looks alright:
English Griffith Close Contact Saddle - Close Contact - New English - Saddles
Used Zaldi All Purpose Saddle - Used English - Saddles
Used Miller's Lancer Saddle - Used English - Saddles
I know that Zaldi are supposedly rather expensive and higher quality saddles, so the $600 seems to be something worth looking at.
As for the other two brands.. I've never heard of them, so they could be junk, although the "Millers Lancers" saddle doesn't looks to be very bad quality. The Griffith, I'm not too sure about