For instance, we assume that he begins to buy it rapidly -> prices will be rapidly rising -> creating expectation that price will rise even more -> most of market makers will change their price for a higher one -> this will lead to a rise in price even more rapidly than Gates buying them -> there are different scenarios could be taken into account from this point:
1. As already mentioned by others, some people might not sell their coins -> shortage of supply and increase in demand leads to a higher price in short run -> later the "Gates" hype will go down and prices will begin to go down making to panic those guys who didn't sell (opportunity profit goes down) with Bill panicking even more as he is now a great looser, making him sell in loss, therefore driving price down even more rapidly than it went up.
2. Imagine that everyone has its price, and as someone suggested someone will sell their coins for $1 trillion (ceteris paribus, meaning we do not consider hyperinflation of usd), therefore Bill will not be able to buy all coins, and eventually it will lead to the same situation as later in #1 scenario.
3. We imagine scenario #2, but Bill has enough money, therefore he buys/gets every bitcoin(he even hacks those wallets, which passwords was forgotten, and even recovers crashed hard-drives with bitcoins, etc). From here there are possible other scenarios:
a.) He holds them to produce shortage in supply and increase in demand to make money, as he thinks, but market realises that the prices Bill was buying for actually are not fair -> therefore prices go down, Bill 'loser" sells at loss.
b.) the same scenario as in 3a, but we imagine that bitcoin community came up with some magical usage of bitcoin, so that it creates unreal demand for bitcoins, price continues to rise -> At some price-levels Bill will begin to cash out, everyone thinks that Bill knew this before, and everyone thinks that he is a smart-ass, W.Buffet is little girl in comparison with Bill.
4. The same scenarios as in 3 (a and b), apart from the fact that buy not Bill but the US government, here we have different approach, imagine that Gov buys not based on financial means but to destroy the whole system, they are printing enough money to buy all bitcoins and not going to sell them at all, not even under possibility of great gain, what do we have from here:
Very big supply of USD, we are very rich and switching to litecoin
-> weaker dollar, we are very happy -> GOv realises that and begins to switch its economy on bitcoins, and revealing the "TRUE AMOUNT" of USD in circulation (admiting that they were covering this information) -> litecoin rises on this information enormously -> we are again very rich
BUT everyone else who was holding fiat money are broke -> therefore GOV and many who became rich begin to donate and air drop money, the so-called 'redistribution of wealth' takes place
