You do realize we didn't always have exchanges or local bitcoins.com, right? And if you want to buy or sell ฿500, there aren't many places to do that easily...
I don't think there are any places you could buy or sell 500btc without causing a major spike or crash in prices.
A private sale would prevent that. See the Silk Road bitcoin auction as an example. Had those 30,000 seized bitcoins been dumped on an exchange, it probably would have eaten through every single buy order and crashed the price to 0 in an instant.
Exactly. Hence the ratings.