A. If the vendor has left the country then your chances of recovering the debt are difficult. The Town Hall will force you to pay up the debt, and will most certainly commence legal proceedings to that effect. The law regarding this tax has been slightly amended and now the buyer is liable for the outstanding amount where the vendor is a non-resident. You need to find out if the seller of the previous sale of your property was a resident for tax purposes or not, and then contest the tax demand with the assistance of a lawyer.
Another possibility is to deposit the tax in the treasury department of the Town Hall together with an explanatory letter claiming a refund of the tax. This is normally done by the overcautious property owners.
It is not altogether clear if the concealment of this hidden charge could qualify as criminal swindle, specially with very recent rulings acquitting vendors falsely stating freedom of charges of the property object of the sale (see news 8-Mar-2000). However, the civil jurisdiction is always open to pursue a refund of the tax together with interests and fees.
I suggest you engage a lawyer to study the different options.