[cheap]

From the official power ball website:

CAN I BUY POWERBALL TICKETS THROUGH THE INTERNET?

No. Start with that answer and then slowly back into "sometimes". Gambling is illegal in the United States. A State has the right to gamble or create a lottery ONLY within its borders. When a game or information about a game cross a state line, or the national border, then it falls into federal jurisdiction. So, some lotteries do sell TO THEIR RESIDENTS through the Internet but not across their state line. Powerball tickets can only legally be purchased at a state lottery sales terminal in the lottery jurisdictions that sells the Powerball game. A lottery can also legally sell tickets on the Internet, but only to persons within its own state. No one can sell lottery tickets by mail or over the Internet across state lines or the U.S. national border. No one except the lottery or their licensed retailers can sell a lottery ticket. No one. Not even us. No one. No, not even that web site. Or that one. You really don't need to send me questions about a specific site. None of them can legally sell lottery tickets across a state border or the U.S. border. No.

Law prevent the SALE (exchange of consideration) of any interest in the outcome of a gambling event. If you are not buying from a licensed retailer in the state or from the state lottery site (within the borders of that state), then you should not expect to be able to collect your prize. There are no regulations of websites that claim to sell tickets or to sell you a "service" to buy and hold tickets for you. Many lotteries believe that they would violated state and federal laws is they paid on those tickets purchased (if actually purchased) by an unlicensed reseller.

You can legally travel to another state, buy a ticket, and bring it back to your home state (at least I believe that no state now has a law forbidding the possession of a lottery).

Now there are dozens and dozens of websites where you can but these tickets online, they require your details and credit card details. So you purchased through an unknown 3rd party website and you won, what would be the chances of that 3rd party paying up?