Brad Templeton points out that by-mail absentee ballots are subject to vote selling. Our secret ballots keep the secret from the vote counters, yes, but they also keep your vote secret from everyone else. That means that nobody can verify how you voted. That means that nobody would reasonably buy your vote because you have no way to prove the vote that you sold. Unless, that is, you voted using an absentee ballot by mail.
The presumption here is that selling your vote is a bad thing. Let's look at it from the other direction. A vote in favor of their candidate is worth money to them. They could buy your vote if they could trust you to the vote you sold. We have worked hard to make that impossible, and yet the desire to buy a vote is still there. They will seek to buy your vote in other ways. They will blanket you with appeals to vote their way. They will have the candidate promise to vote for things you want (do you remember what he promised last time? Did he deliver?)
Vote-buying has two good effects. One, it recognizes the reality that commercial interests stand to benefit from your vote, and why shouldn't you share in the gain? Two, it forces the party buying the votes to actually expend money. Once that money is spent, it's gone. So, they have to think very hard about the value of your vote to them. Perhaps they would do better to spend that money on something else.
UPDATE 11/7: TM Lutas points out that if vote buying were legal, then the cheapest way to take over a country would be to simply buy the votes needed. No war or coup needed. Of course, politicians are currently legally buying our votes using OUR OWN TAX DOLLARS. "Vote for me and I'll do X, Y, and Z for you!" Yeah, right, you and what bank?
I would argue that our government has already been taken over by a hostile power in exactly the manner TM Lutas fears, and worse, they used our own money to do it.