Post some nice pictures, start with a reasonable price and be prepared for low ball offers. It's a fickle market and really comes down to offering a horn at the same time that someone with cash to spare is looking for one.
You'll determine it's selling price (if not its value) when you and a buyer exchange funds and horn.
The Ref 54 alto is one of their better playing altos and the kook' engraving is nice.
From Kesslermusic.com:
"The Kookaburra Reference series is #2 of 5 the "Bird Tribute" Series. Released in 2006, the Kookaburra honors Australian saxophonists with the beautifully engraved Kookaburra birds.
The horn is a standard Reference 54 alto, arguably one of the best saxophones ever made and currently the top end model from Selmer Paris.
The "Kookaburra Standard" is set at a maximum production of only 330 for the entire USA. Once that number has been hit, this collector's edition horn will never be made again."
Selling price of the Kook' 54 alto was about $5k at the time, a new Ref 54 alto (without special engraving) is currently around $5200. You might start at $4k and see - if it sells fast, you started too low; if it doesn't move, the price is too high.
G'luck!