Offer him $2,200. These don't sell on Craigslist locally like they do on e-bay. A new box from McIntosh is about $100. New Glass is about $150-$200. More than likely a couple of light bulbs will be out too. And then there's the remote. Add in labor for installing glass and bulbs. Then factor the discount the seller will give you for the convenience of local pick-up with cash and no recourse.
I bought a used MA6900 a couple of years ago. I cracked the glass when I was lifting it from the box by myself. NEVER pick it up by a corner as the chassis can flex and crack the glass (yes, I learned the hard way). It was a fantastic unit. I got a great deal on it as it was part of a package (divorced wife got the equipment and wanted to piss off ex-husband!). Very powerful and clean. I personally think the $3,500 - $4,000 is way overpriced for an integrated unit. But if you can get it for $2,500 or less, you've got a great deal.