US $1 billion?

When I was in Singapore, I found this note and several versions of it including the million dollar. These sold for $2 each. I am not sure of the legality of selling these notes or even buying them.

US Dollars are currently printed denominations are $1, $2, $5, $10, $20, $50, and $100. Notes above the $100 denomination ceased being printed in 1946 and were officially withdrawn from circulation in 1969. These notes were used primarily in inter-bank transactions or by organized crime; it was the latter usage that prompted President Richard Nixon to issue an executive order in 1969 halting their use. With the advent of electronic banking, they became less necessary. Notes in denominations of $500, $1,000, $5,000, $10,000, and $100,000 were all produced at one time but never the $1,000,000 or the $1,000,000,000 which is reproduced above.