If you look through the various Spanish for-sale-by-owner websites you will quickly find people advertising their VPO flats for sale with a note pointing out that the property is, in fact, a VPO and therefore most of the price will have to be paid in black money. These people are surprisingly open about their illegal scheming.
It is also fairly common for sellers to exchange contracts on their VPO’s without mentioning to the buyer that the property is a VPO. Of course, the buyer will quickly discover the truth when he attempts to raise a mortgage and the bank refuses.
Normally, you can discover if a property is a VPO by asking for a nota simple at the local property registry. However, beware – because VPO’s come in many shapes and flavours and it dangerous to generalise about the resale conditions.