There are three people are well-placed to say how much money is 'market value': 1) The owners themselves, by self-declaring asset values for taxation purposes

There are three people are well-placed to say how much money is 'market value': 1) The owners themselves, by self-declaring asset values for taxation purposes (on pain of having them vulnerable to eminent domain seizure if they under-declare) 2) Independent assessors who do value assessment for property tax purposes already 3) The sellers that granted the owners possession for a certain value