The law can be vague, contradictory, inconsistently implemented and arbitrarily enforced. On top of that promising start, most owners are blissfully unaware that such laws exist.
A powerful hotel lobby that sees holiday rentals as a threat is usually behind these laws, but given the glut of homes for sale and the severe slump of the Spanish economy, the state should be bending over backwards at all levels to encourage people to buy, which implies making tourist rentals as easy as possible.
But no. Instead they are issuing stunned owners with fines of up to €30,000 for renting out their holiday homes to tourists. If you engage in holiday lettings, you are taking a big risk and there are quite a number of owners prepaired to take that risk.
Its very easy to trace the majority of owners who advertise on the web just by asking the address of the property and were payment is to be sent,it's not rocket science.