For example, by the mid 80's, the cost of a small cooperative apartment in Moscow was a couple of thousands rubles. How that money could be earned in the state where an average family monthly income was about 200 - 300 rubles was a mystery.
Do you want to say that the price of realty in large cities in other countries is smaller? I heard stories about people repaying their realty credits for 20 and 30 years.

If you disobey the state (aka break the law), that apartment would be taken from you by the state (aka the confiscation of the belongings).
Confiscation was only possible as a penal measure for crimes against property such as theft, robbery, fraud, extortion etc.

you would be obliged to liquidate your private property only to compensate another party for the amount decided by the court and the rest of the value of the property is yours
In the United States one can easily get life sentence for crimes against property. Confiscation is evidently more humane.

And it some cases it was very difficult to estimate the damage.
Indeed. That's why the state made it impossible to become rich through taking others' property. In today's Russia one can steal billions, hide them, get imprisoned for some 5-6 years and then continue a billionaire's lifestyle.

For example, if you secretly made private business producing something (e.g. the food) and selling it privately beyond the norms allowed by the state, you made the proprietary damage to "the people" since the "extra food" that you produced belongs to "the people" and you stole that from "the people" and it's very difficult to estimate the price. Hence, the confiscation of everything (with the subsequent imprisonment).
In any western state if you secretly make business and do not pay taxes or, say, produce counterfeit goods, you will be imprisoned as well.