Transparency in estate values

The only people who really know are the buyer, the seller and the notaire(s) who handled the sale. You'd have to ask them. Assuming it is a private sale, (as opposed to an auction) there is no public access to this type of information in Belgium.

You can't - it's a major downside of the property market in Belgium.

The best you can do is check the ads for similar-sized properties in the area to get a rough idea and have a word with friendly neighbours.

I understand perfectly that knowing the selling price of a property can help you assess what a similar property in the same area might be worth.
I don't understand why you, a total stranger, feels that you should have the RIGHT to know how much I sold my house for.
What would you be happy with total strangers knowing about your financial dealings? And, perhaps more relevantly, with family members and close friends having the RIGHT to know?

Check zimmo.be
It gives you an indication of property sold in the last 6 to 12 months
Search property type and you will see the estimated price of similar properties

The standard deviation of the price is very high between the properties- but it gives you a rough idea

Privacy doesn't come into it. Buying a house is as much a public act as a planning application or a will. Civilised (and transparent) societies recognise that and allow access for the general public.