You clearly have no idea what you are doing. I would suggest you wait a while before you actually understand site value, and how to monetize a site properly. If someone is seriously interested in your website and is willing to go above $200k, there is some serious value attached to your website. They are very interested, clearly, and will not give up as they probably flip websites like yours all the time.
Start building email lists from your visitors if you can, try out 20-30 different CPA offers for display campaigns, split-test as much as fucking possible, and evaluate your results over a 30 day period before even CONSIDERING accepting the $200k offer. If anything, that offer will still be on the table in 30 days and you will have a better idea if your site can be monetized enough to keep it.
Sounds to me like you have a website that took off by random chance. Figure out your demo and monetize accordingly. If you think you can't handle all of the above your self, seriously consider hiring someone to help you monetize your site and pay them well so they do a decent job.