Basic land sales contracts are the simplest real estate transaction out there. Its one of the things I love about land. Generally speaking, RE agents take 10% (because the sale price is less than a house) on a land sale so that's a bit bitey. If they can avoid it, and the buyer is willing to sign a simple contract without a bunch of complicated clauses, they may not really need an agent.
Normally, I'd agree that unsolicited letters aren't going to maximize what the land is worth, but I've now seen enough in real estate to know it's more complicated than that. There are so many companies or people out there with ridiculous amounts of money that they could be looking to gobble up property in the area with the intent of development years down the road. With enough money and a long time horizon, a buyer now may know that it's money in the bank 10 or 20 years from now as growth hits that area and it becomes prime for development. That's just one scenario of course.
I guess it really depends on what everyone wants from the property. Do they want to hold for a long time, looking for the big payout? Maybe no one needs the money and they're willing to pass it on to heirs until a developer comes knocking. Though, being split into four pieces already, things may get dicey if the ownership becomes any more fractured. Maybe taking a nice profit now is not a bad end game.
If there are two parties that want out, perhaps this offer is a great way to establish some kind of current market value and the other two that want to hang on can buy them out of their half.
Food for thought!