In Australia, at least, anyone can apply to vote by mail (which is very useful for the elderly or super-busy), and voting by mail is the only way to vote in things like local council elections. Voting is also compulsory. It seems to work fairly well; I've never heard of a vote-purchasing scandal, and any amount of vote purchasing which was on a large enough scale to influence an election result would be almost guaranteed to be leaked by somebody.
If you can't guarantee that the person is alone, then they can be coerced into voting a specific way. If you can't guarantee that the person isn't observed, then the person can sell their vote.
Every vote counts, for sure, but when your potential voting population is in the millions it'd become very difficult to cover up a vote-at-gunpoint scheme from the police. Similar with voting for cash, the chances of being able to conceal such a scheme–if it grows to a size where it can influence the outcome of an election–are small. The lack of needing to visit a polling station should also increase the number of people voting, which may work to counter-act the influence of the small number of votes which are compromised without alerting police.
It's a massive hole on paper, for sure, but it remains to be seen whether it would actually result in any visible influence if it were enacted in practise. A much more effective way to purchase votes seems to be buying a news-media outlet–and it's legal to boot.