Should a person be allowed to sell their votes to the highest bidder? As stated by
Victoria, even if this practice were legal, the problem here is verification. How would the bidder know that that purchased vote was in fact cast their way? A couple of ways I could see the verification problem being solved:
Proxy voting: if the law would permit "proxy" voting, then a purchaser could purchase the right to cast another's ballot by proxy.
Edited to add: It occurs to me that an absentee ballot would work here. You could sign your absentee ballot and then mail it to the proxy center to be filled out and voted for you...
Statistical counting: A vote purchaser could offer a "pot" for a block of votes to be cast their way. A vote seller would receive payment if it can be determined that the net block of vote went the purchaser's way. Since there's a margin of error for determining this, only a percentage of vote sellers would receive payment.
Regardless, I believe selling votes is illegal. I know that during the 2000 election there were a number of vote trading sites: see
Wired article. Ahh...those were the days, weren't they

....
How much of an effect would that have on elections? Well, I could see a similar effect to how the California proposition process works. It takes
signatures from 5% of the number of people who voted in the previous election to get an initiative on the ballot AND the signatures must be collected within a 150 day interval. There were ~12.5 million votes in California in 2004. That means for a prop to get on the ballot one would need to collect ~4100 signatures
per day. Thus, you need an army of people to collect signatures. The only way to do this is to
pay them. In other words,
only the special interest lobbies with a lot of money can afford to get an initiative on the ballot in California.
Thus, I conclude that, for vote selling, the net effect on elections would be roughly the same as today. Special interests still determine the course and debate of the electorate. But hey! At least the voters can get a piece of the action and not just the politicians...