From my many years of experience in business I can say that anytime you have a serious offer on the table you have to decide if you are willing to risk going for door number two.
You can always point to cases where people didn't sell out and ended up better but with business you have to look at the downside not just the upside. Over the years that is the biggest mistake that I have seen people make in negotiation.
I'll give you an example with domains. I'm in the process of buying a domain for a startup and the startup will pay $50,000 maybe even $100,000 for the domain. But the owner won't even engage preferring to shoot for the stars and thinks he will get a million. He won't even state a price. At this point we will cut bait and find another name. I know fully well this business (I also own names) and I know that this domain has about a 2% chance of selling for anywhere near that amount in coming years. I have domains that I've held for 15 years and have never gotten a single offer on. I've had domains that I've held for 15 years that I've sold for big money. Anytime someone comes along with a serious offer I focus on how long it will take until the next offer will come in - if ever. Business and gambling are two different things.