If someone offered you money to purchase your vote, would you sell it? Legality aside?
If they wanted to pay you to in exchange for your word that you would vote accordingly, they would have no way to know if you kept your word. The secret ballot voting mechanism in the United States ensures that.
If the vote buyer wanted to be sure that you voted for the candidate for whom you were paid, they could make sure that you appropriately fill out an absentee ballot. They could withhold payment until the form was completed and returned or some other form of watching your complete the absentee ballot.
Due to legality issues (and my small blog readership), I don't foresee a huge surge in vote purchasing in the near future. However, if you want to "sell" your vote for the "wrong" candidate, don't offer to do it via absentee ballot.
Credit for this idea goes to Pam, Whitney, and the rest of my Public Choice class.