An option considered in one state (Utah?) was to allow mail-in voting, but the mailed ballot could be invalidated if you stopped into a polling place and voted. It adds some complexity but reduces the absolute chance of coercion.
Only you know who you actually voted for. That's the point.
I'd guess mail-in ballots increase participation at the expense of some coerced ballots. Surely the net outcome is positive. On top of that, it helps to remove the chance for the use of intimidation at the polling stations. I remember that being an issue last election.
Somebody else mentioned that some areas have a system where if you show up in person, your mailed ballot is invalidated. That seems like a good safeguard.
The bottom line for me is that I think the option of mailing ballots increases participation. There may be an increased number in coerced ballots as well, but I think the net result is postitive.
Canceling mailing or e-voting ballots when voting in person can be countered by requiring that the person hand over his ID card for the vote duration.
The idea that a vote might have been bought is a dangerous one. It's dangerous even if votes buying are not proven. I'm still not convinced about e-voting for this reason. There should be absolutely no doubt on the sincerity of a vote.
In most states this is illegal. In, GA, the SOS posted a reminder. (follied by the President's picture)
Long lines or inability to get to the polls on time has contributed to my not voting. Plus it's great to be able to sit with a ballot over the course of a night or two and really think about what you are going to choose. Being at the polls with people waiting behind you applies some pressure to pick the fastest option - ie choose all Democrats or all Republicans (the machine I was on had a shortcut button for that).
Last election I voted in Texas as people voted quickly. It seems like they either are only voting for one or two races or they are just pressing the button that selects all Republican (I live in a red area) candidates.
You wouldn't want your colleagues at Google knowing you voted for Trump now would you...?
Husbands can coerce their wives into registering for a postal ballot and then vote on their behalf. Religious leaders can coerce their followers into registering for a postal ballot and handing it over to a chosen candidate. A corrupt care home worker could register dozens of residents for postal voting, then sell their ballot papers. Migrants with a poor grasp of the English language can be tricked into giving up their ballot paper. Poor people can be offered cash for their ballot paper. Postal ballots can be stolen from postboxes and sorting offices.
All of these frauds have taken place in the UK; local election results have been nullified on more than one occasion due to endemic fraud.
http://www.telegraph.co.uk/news/uknews/law-and-order/1156001...
http://news.bbc.co.uk/1/hi/england/west_midlands/4406575.stm
http://www.bbc.co.uk/news/uk-england-london-32428648
http://news.bbc.co.uk/1/hi/england/berkshire/7302809.stm
https://www.electoralcommission.org.uk/__data/assets/pdf_fil...
It's much easier to prevent that kind of thing rather than repair the damage after the fact, similar to the problems we've seen with gerrymandering.