For a like new condition, anything under $500 is a steal!  Definitely buy it - and if you don't like it, I'll buy it off you for $475 - so you can make a quick $50 if you don't like it (I'm serious - email me if you want).  

Pre or post doesn't matter that much - since the only difference is the flash supressor that the pre-ban has, but the pre-bans sometimes have a few bugs that the post-bans don't have.  Don't worry if it's pre-ban  - it's worth a little more, and Bushmaster will gladly help with the bugs.  Either way, it's a great deal.

I bought one a little over half a year ago for $400 (like new condition as well) and it was the best gun bargain I have EVER made.  It was for sale on this board, and some people still hate me for having been the lucky guy that got it.  [:D]

Keep in mind it will eventually cost you a little more, since you WILL need optics for it (the fixed sights suck) and you might want to have some modifications made by a gunsmit at some point.

By all means buy it if its condition warrants.  Search topics on this site (try all forums): bullpup, M17, M17S.  Also go to [url]www.google.com[/url], use keyword M17S.  You'll get some ideas about how some folks feel about their M17S's, and their modifications to them...  Good luck!