How big of a problem is selling votes? I would think that anything on a large enough scale to be effective would be extremely difficult to get away with.

I know when my absentee ballot was lost in the mail the worker at the registrar's office just marked it as spoiled in the computer and gave me a new one. Multiple submits with the last one counting seems like it would solve the problem. You could add a count-this-and-not-future-ballots checkbox if you'd like to let someone submit a last minute poser-ballot to make the buyer/intimidator happy.

On a different topic, I think it's outrageous that absentee ballots require stamps. Mine even requires two stamps to actually have correct postage.