$600 CZeiss 16-80 for A57?!
$600 is about the going rate for a used example in great condition (complete with all accessories). I would make sure to have a good return policy and the possibility to check the lens thoroughly.
I went through a couple of these and while they are optically excellent (contrast, colors, sharpness), the mechanical build is rather weak. Issues I (and others) encountered:
1. MF clutch is fragile, focus ring locks up which could cause/accelerate 2.
2. AF gears strip (need to change complete focus tube assembly), expensive repair
3. focus jumps during zooming (mine did around 24mm), needs adjusting
4. viewfinder image shifts during zooming, needs adjusting
5. zoom creep
Most of these issues are probably limited to older lenses (1st batches), especially the AF gear design was modified but there is no safe way of telling how old a Sony lens is or when design changes were put in production. If you have a good example it beats the 18-250 throughout the range shared, IQ and AF speed are superior.