The paper based system guards against vote buying through the guarantee of secrecy. I go into a booth on my own, make my mark(s) fold my paper, and put it in a locked box. No one but me knows how I voted.
If I go in with someone else, the officials will take action.
You can try to buy my vote, but I can just take your money and vote however I want.
If I can vote from wherever I like, then someone can stand over my shoulder and watch me vote, then only pay for it if I do as I'm told.
Sending a vote buyer knocking on doors and watching people vote when they happen to be voting is not very scalable. However, party activists and pollsters already do the first part. Adding the second isn't a massive leap.
However, there are situations that might scale better.
A business owner or manager might strongly persuade their workforce into voting the "right" way, by getting them to vote at work. This would scale quite well in a non-office environment, where you might suggest to your workers to come and use one of the few computers you have (for their own convenience, of course).
Another option might be some kind of party, where you invite people to come and vote for your candidate in exchange for payment. This could work quite well to gain the votes of the poorest in society, who may otherwise find it difficult to vote (no internet at home, can't easily get to the polling station).
Remember, those who care enough and are decisive enough to vote early in the day, are not the target of vote buyers. The people whose votes you can buy are those who wee possibly thinking of not voting at all, or those yet to decide as the polls are closing.
Think about the scalability of the other side. At the moment, you have a few people per few thousand voters sitting in the designated room for those voters, watching out for violations.
Both of these intimidation/buying activities would be illegal, and would certainly not be endorsed by the actual party in question, but by rogue individual supporters of the party. However, detection and prosecution of offenders will be far more difficult and time consuming.
Having to detect and prosecute these irregularities after-the-fact would also mean that elections may have to be invalidated and redone (possibly ad-infinitum) when a violation has happened, if you just remove all the illegal votes and recount, then what about all those voters who did vote with their conscience, but just happened to do so in the wrong place. Then what do you do about the actions of the illegally elected in the interim?
If I go in with someone else, the officials will take action.
You can try to buy my vote, but I can just take your money and vote however I want.
If I can vote from wherever I like, then someone can stand over my shoulder and watch me vote, then only pay for it if I do as I'm told.