There isn't always a buyer if theres a seller.
For example would you purchase something that will be worth less in the future? And to finance this you will require to go into debt?

For housing the danger comes from point been reached of a glut in the market.
The only people who matter are those with capital, all others are irrelevant.
Everyone wants property but only those with capital can afford it.
Some of that is based on equity. And the vast majority based on debt.
When a fall starts to occur on house prices those who borrowed cannot sell for the loss, or if they become unable to fulfill their debt repayments then the bank reposssess the house.

PJ your incorrect in that assessment. Housing crashes do occur, property has no inherent value, it has value because we give it value. There rare crashes and I doubt the US will have one.
But the only thing that can really cause one is when a downturn hits the banks cut of credit.
This shakes confidence more further reducing prices.

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"Better to die on you feet than live on your knees" - Emilio Zapata