I've seen that craigslist ad. I think the guy selling it is a member of the Puget Sound Aquarium Society.
With $6000, this is what I would do:
If its my first reef tank, then I would rather build my own tank because a 500g is really hard work (even for an experienced hobbyist)
If 500g is considered to be my second or third tank, then I would change my mind and build my own tank again.
The deal has a lot of equipment that I would not use. I wouldn't want to even bother reselling what I don't need (time consuming).