While it's true that law enforcement has a role to play here, if you're being paid, you probably aren't going to report on yourself.
> individual votes have almost no value on their own
That's probably true in a national election, but it might not be true in e.g. a mayoral race.
I have offered to sell my vote to anyone in my area (a swing area of New Hampshire) who wants another vote for their side in any election at any level. I've never even set a price.
Not one person has ever offered a cent for it.
I mention this just by way of making the point that small-scale vote buying, which would be the hardest kind to detect, isn't happening.
Theoretically at least, detecting and preventing vote-buying at a scale significant enough to change the outcome of an election should be much easier.
Would-be buyers have to get word out to enough people for it to matter, but if most people assume this is unethical (or even just undesirable), it shouldn't be hard to get nearly everyone else to be willing to rat out the buyers.
I don't think the problem is nearly as hard to solve as you're thinking it is.
The odds of your vote swinging an election at any level are lower than the risk of being struck by lightning twice. Nobody wants to buy one vote. They need to buy hundreds or thousands before it will matter, and that makes them easier to catch.
So your result is the expected one. But the outcome would not be the same with most types of harebrained "let's just vote with our phones" that people come up with regularly.
This is also why it is in most countries illegal to take photos or film around the voter booth.
A photo of me with my completed ballot, in the voting booth, has always been part of the offer, since that was the same year I finally quit bothering to cast my own ballot.
[0] https://www.nbcnews.com/news/us-news/federal-appeals-court-r...
But I believe major point is still the same. Nobody knows if that's the actual posted ballot. You could easily take several and only cast one.
It is my personal belief that votes absolutely carry a market value, they're just very hard to monetize. (Which is usually regarded to be a virtue.)
My assumptions are that 1) individual votes have almost no value on their own, so coercion or vote selling should be limited by low appeal, and 2) we still have a justice system, if someone forces you to vote a certain way, report it. Buying any significant number of votes would be exceedingly difficult to keep secret. Especially given how little compensation could be offered for them.