Some legit stuff is going on as well:
Lots of players have multi billions of gold that they are stuck with. It's impossible to sell any meaningful amount of gold on the RMAH in a short period of time. So to convert your billions to real $$ in a legit way you have two options:
1) buy BIS gear (for example 6CC Mempos) on the GAH for 2 Billion and sell them on the RMAH for cash. If you can sell the item you get a lot more money than the 2nd way.
2) Buy Radiant Stars on the GAH and sell them on the RMAH for cash $$.
I could easily flip 1 billion gold for cash in minutes this way.. The downside is that you get a lot lot less. 1 billion gold would buy about 50 Radiant Stars. @ $1.25 each you'd get about $37 in your PayPal account after all the fees.
Note that the amount of money you get (~$35) is coincidentally the price of a billion gold from chinese gold farmers. No wait, that's not a coincidence, that's a tightly linked relationship.