A law against selling any house cheaper than it was bought for, what consequences would that have?
That would be really, really bad.

Any house that loses value will be unsellable, and thus virtually worthless.

Most people living in such a house would be prevented from moving. They cannot sell it, since no one wants to buy an overpriced house, and they cannot afford buying another house with their capital already tied up in the current house. 

A black market would appear, with people giving money back under the table. "Buy my house for 4M SEK and I'll refund 1M SEK under the table."

I can tell you one thing it won't do. It won't stop prices from ever falling.