Unfortunately there is no 100% guaranteed solution Sarah. What can be obtained from the Tax Office is a “valoración vinculante” by which they acknowledge the value of a property and they cannot fine you if you purchase at this price or above it. This value may be a multipleofthe cadastral value,which will be below the current market value. The tax office is binded by this value they themselves give you.
Clearly it is the lawyer’s responsibility to warn the purchaser about this risk, not the agents job. However, for REA’s their reputation is always important and no one wants unhappy clients coming to forums such as this one bickering because they feel deceited. That’s why it would be a good idea to write short paragraph a a disclaimer and make the client sign it acknowledging this risk. I’m sure the lawyer will do the same on his side.
The Tax Office isn’t going to always fine you for this but there is a risk and they can do it. As long as the purchaser knows there is a risk and he acknowledges it then they cannot complain later on. Many people are going to buy properties from desperate vendors at bargain values in these years so this issue will become common.