"Given that Bezos is currently worth $193.5bn, according to Forbes' latest estimate..."
Not to get in the way of a tasty bootnote, but you do know this doesn't mean he actually *has* $193.5b, right? Nor could he obtain that much cash by liquidating his assets. These "net worth" figures are based on a blend of guesswork and regulatory filings, and even where it's known for certain that someone has X shares in some listed company, the people compiling these figures simply multiply X by some recent closing price and call it good. In reality, order books are not infinitely deep, and anyone trying to sell some large X number of shares will find that the price he gets for more than the first few thousand quickly approaches zero.
As a result, no one who wants to unload a large ownership interest does so on the public markets, which also means the actual price paid (almost always quite a lot lower) has to be negotiated with one of the small number of capable buyers. Furthermore, most of the people who end up on these lists have huge concentrations in individual companies. The knowledge that they want to sell makes potential buyers wary -- a product of a market inefficiency called information asymmetry. You can read about all of these things in any introductory secondary school economics text.
I'm sure Mr Bezos has plenty of money all the same, but the idea that someone automatically must have no trouble finding $500m in cash just because some magazine published an inflated estimate of his nominal net worth is pretty silly. The amount of cash someone has, or could readily access, is at most weakly related to the nominal market value of his illiquid assets. That's exactly why some assets command a "liquidity premium" -- and assets like real estate, helicopters, and shares in an overvalued Internet retailer are not among them. If Mr Bezos's wealth consisted mainly of gold, Treasuries, gilts, cash, and the like, it would be a different story. It doesn't, so whether you rely on Forbes to keep score or as a source of outrage, you need to know that those numbers are largely meaningless.