The ability to sell one's vote is a minor problem compared to things like fraud and low turnout. More obviously, it's far easier to buy a politician who represents 50,000 voters than to buy thousands of votes to buy your favorite politician into office. (And a permanent absentee voter like myself, can always sell his vote anyway - and this isn't causing and horrendous problems that I'm aware of here in California.)

Say you implement something like Rivest's ThreeBallot system, or Chaum's Punchscan system. You have virtually fraud-free elections, and high turnout. A few people who may sell their votes is hardly your major concern.