No sale of property in Spain is valid without the presence of a notario.
A Spanish notary is possibly one of the most comfortable (and profitable) professions in Spain. His (or her, although most Spanish notaries are male) role in property deals is to guarantee the legality of the sale, to certify the presence of all the papers and people required by law to be present in any property deal, to ensure that property buyers or sellers who do not speak Spanish have a representative with them to translate proceedings, and generally to preside over the signing of the property sales contract and to register the new deeds.
Whereas a search for houses for sale in Spain begins at an estate agent, it ends in the notary's office. Both the property buyer and vendor (or their legal representatives) must attend the signing of the contract, and the estate agent (if there is one involved) and lawyers are usually also present.
If a Spanish bank is involved in the property deal, then the bank representative also attends the meeting to confirm the mortgage which is also signed infront of the notary.
If, as is often the case, the value appearing on the deeds of the property being sold is less than the overall price of the sale, the notary generally turns a blind eye.