Hi Jackpete
You are confusing a lot of separate issues here.
If I have a property worth €500k and I sell it for €200k, I am effectively giving a gift to the buyer of €300k and they will be subject to Capital Acquisitions Tax on it.
If I put a property for sale and ask for €500k, but the best I can get for it is €400k, that is the value of the property and there is no CAT.
Revenue will ask for a valuation if they believe that it is being sold at undervalue. But it's much more difficult to assess the current value in today's market.
Brendan