> But my sense was that it's not that you actually have to sell immediately but more like someone can offer to buy at $X and then you either have to sell or $X

> But my sense was that it's not that you actually have to sell immediately but more like someone can offer to buy at $X and then you either have to sell or $X becomes the new declared value for future tax payments. That can easily be maliciously abused too, using it to drive someone's taxes up until they're forced to sell.