Have you tried? https://flippa.com/ http://www.dealasite.com/ http://www.websitebroker.com/ http://sitepointmarket.com/

At least Flippa was mentioned here some times. I don't know how theses sites deal with as 650k/year site, but you are trying many options, I guess...

There is also the old discussion about a site value: http://news.ycombinator.com/item?id=355627

PS. You could have added "Ask HN:" in your title. That would probably attract more people to answer you.