It certainly is illegal!
I have just reread the article and it states that it is too.
And I definitely do not advocate it either.
The risks of paying anything in black or ‘dinero negro’ are clearly stated.
The article simply mentions that a vast majority of property sales between two Spaniards involve some ‘cash under the table’ and a Spanish vendor may even try to demand it as part of the sale.
However, since the Marbella court cases and the corrupt Julian Muñoz hit the headlines by being sent to prison, this is changing and people are far more cautious and there is a growing reluctance to pay anything in black.
I certainly wouldn’t give my fellow countrymen or anyone else come to that such advice and I would appreciate it if you reread the article and got back to me if you still think it comes over that way. The article was written to be nothing more than a quick and light-hearted aid to buying a previous-owned property.