Think about it. If they were offering you one billion dollars, would you still be unsure?
If your answer is "I may still be in two minds about selling", then don't sell.
If your answer is "I would sell in a heartbeat", then lower that limit, i.e. would you sell for a hundred million? 10 million? 5 million?
Put a price on what you would be willing to sell for without hesitation. Then evaluate the difference between that amount and the offer. If it is less than 25%, sell. If it is over 50%, don't. In between 25% and 50%, I don't know - but may you don't even fall in that window.
(I've somewhat modified the quote)