That would be the whole point. It is easy to value a public company because shares are sold thousands of times a day. This establishes a current market value. When companies go through acquisitions, the fair market value is determined by how much another company is willing to pay. The accounting nerds go through this valuation process and end up with “goodwill”. IMO, the fair value is the price when it exchanges hands. This is why DJT is able to say his properties are worthy X and someone else say they are worth Y. There is no clearing house for the real estate properties.
Valuing a company is not easy. Unlike your car, there are many out there on the market with similar options, similar miles, similar condition, etc so you can determine the value with some level of precision. A house is similar. Similar size, condition, sqft, school district etc.
If you take a company like Facebook and try to assign a value, you can have 100 experts come up with 100 different values.