That technique leaves quite a lot of evidence around.

Not only do you need every voter you buy the vote from to never speak up about it, but you also need to somehow hide all the mailing of absentee ballots, hide where you mail them back from, and your money needs to be clean and untraceable to yourself. AND all of that needs to happen right around election time.

But hacking an electronic machine can be done by a few people making changes to a warehouse of voting machines by impersonating a janitor 2 years before the election. And that's not hypothetical, 100% of voting machines that have been in the hands of hackers have been exploited to change votes. I myself used to sneak into the room where they kept the voting machines in my high school when I was younger.

"online" voting is even easier to hack. You could attack the users by buying a botnet and have it submit or change the votes on the client machines before they cast them, you could attack the servers via things like evil maid attacks on the server hardware, "normal" exploits on their software to gain access to the systems, even just paying the sysadmin that maintains it a few hundred million dollars to look the other way for a few days.

Which is exactly the reason why absentee voting should be an exception. Every single person who takes an absentee ballot purely out of convenience is hurting democracy, even if only very slightly. And it's not just plain out vote buying, there are also various forms of soft, even unintended coercion that encroach on a voter who votes outside of the enforced privacy of a booth. If it was possible to run statistics on this I'm quite sure that there would be statistical evidence of spouses voting more alike when voting at home than when voting alone in the booth.

What you describe is considered election fraud and is illegal in the US.

see 18 U.S. Code § 597: https://www.law.cornell.edu/uscode/text/18/597

Absentee ballot or any other real-world interaction dramatically increases price per vote and risks.

With electronic vote, both reward and verification can be done in untraceable way. Basically you could sell your credentials for Monero on some Onion site and vote will be done on your behalf. Low risk for both parties and plausable deniability for seller (OMG, I was hacked!!!).

I have been playing with that idea, as some kind of activist political stunt but I am pretty sure my country has limitations in place to prevent/forbid the selling of vote.

Try it. They'll defraud you by overriding the vote in person.