Ask sedo dot com that what it could be worth and sell there if you want to sell after estimation. or display that your website is for sale on your website so someone may buy it.
I don't see any reason to sell such a site which generate a huge amount of money. Just keep it and don't sell it.
You are doing awesome-ly well for traffic generation, but maybe your monetization techniques need to be changed a little.
Some buyers would only base the value of a website on the site's income. It should also include the value of the domain, the value of the content and effort of how the site was established.
If you have the earning and traffic proofs than the site will easily sell for 6000K + easily on flippa. However, i would never sell such a website even if the earning is decreasing. I can still aspect $400K from it next year.
Knowing the right time to get out is crucial to your long term success... It may have peaked. You never know. Every seller has a different reason for getting out. Health, boredom, retirement...