Sure, but where did the money come from to pay the tax bill? Pre IPO you may find it difficult or impossible to sell any of the stock you now own, so you'll be

Sure, but where did the money come from to pay the tax bill? Pre IPO you may find it difficult or impossible to sell any of the stock you now own, so you'll be out of pocket from somewhere else. This gets less and less practical for most people as the difference between strike price and value increases.