Sure votes are bought offline too. But it's much more difficult.
The people who sell votes don't care to "change their mind". They get their reward and they're happy with that. People who would bother to change their mind, don't sell their votes in the first place.
The usual scheme is, buyers pick up sellers and drive them to polling places. Police is usually on the look out for cars that frequent the polling stations. Or park just around the corner. The sellers usually are homeless-ish/addicts/etc. Large amount of such people raises awareness. Sometimes approaching such people and asking what's up is enough. They don't bother to lie and just tell they sold their vote to some guy in black BMW for €xx.
Now if online voting was allowed, the buyers could just take the seller's signature device and do voting himself. 100% the vote is correct. No possibility to get caught at polling station. The sellers don't hang around the polling station, thus less chance to raise awareness.
In addition to that, buyers could buy the device en masse from the addicts/homeless/etc and have a ready-to-use voting farm.
All in all, yes offline vote buying happens and it's not rocket science. But online vote buying would be much easier, quicker and less chances to get caught.
As I understand it, there's a similar scheme in Oregon's vote-by-mail mechanism. You can buy someone's mail-in ballot, vote the way you want, and have them sign and submit it.
"could just take the seller's signature device and do voting himself"
That's identity thieft, which should be punishable by a few years behind bars.
Also, make those signature devices tethered to receiving whatever equivalent of Social Security payments, and addicts/homeless will not let anybody lay a finger on it.
Taking a photo of your ballot paper is a criminal offence (at least where I am) and one which can only be committed at a specific physical location and time. In other words, it's a crime with a very low payoff (an individual vote can't be worth very much money), and a relatively high risk of detection.
The risk of getting caught taking a screenie in the security of your own home with no pollwatchers and election officials around is much, much, lower.
You can take a photo of the ballot with the "correct" box marked and then spoil it (eg. by marking all boxes). That's what I'd do if I was coerced to vote for some candidate.
If anything, online voting system can let you "change your mind" later, robbing buyer of vote they paid for.