What about sinking 3 2x4s into the ground and nailing a 4x8 sheet of plywood with a tastefully painted sign indicating the property is not for sale?

It won't stop everyone but any realtor doing due diligence will likely see it. If is lasts long enough, it will show up on Google street view as well.

A motivated attacker need only don a green safety vest and hard hat, then roll up with a white pickup truck, place some orange safety cones and take down the sign with a chainsaw.

The point is that nearly all of the people doing this don't even live in the country where the land is being sold from. A simple sign would probably be quite effective

True, but you can still do a confused deputy attack. The fraudster hires a property manager, informs them that they would like to remove the sign because they wish the list the property for sale. Either that or they con a realtor they're working with into doing it. The unknowing realtor, eager for the commission, knows a guy who can take it down.

There's always something that can happen in any scenario. Social engineering, hiring locals, deeper forms of identity theft, or worse. The possibilities never hit 0, they just become a lot less profitable (and a lot riskier) a scam to try to run.

Yes, locks aren’t there to prevent the determined thief. They are for the 99% of other opportunists that will move on to an easier target immediately when they see your lock is harder to defeat

For those like me who didn't know: https://en.wikipedia.org/wiki/Confused_deputy_problem

Who's paying for it? Are they working for free?

The realtor might pay for it or even do it themselves. It would take 5 minutes with a reciprocating saw. Or the scammer tells the realtor "never mind that" and the realtor tells the buyer.

No, but paying someone $300 is cheap when you hope to get a check for several hundred thousand in a few months. (even if the scam is only to get the earnest money that is still a $300 investment for the final thousand or two you make - with very little work)

That's a lot of work plus money transfer paper trail for something like this.

Note that in the article, the author says how the scammers do everything to avoid having to show up in person. That's because they are in a different country and try to commit the scam without setting foot in the US.