Many buyers are asking this question. For many years it has been normal practice when buying in Spain for a buyer to be requested to pay over a sum of money to remove the property from marketing activity until such a time as a purchase contract can be signed. These sums range between 3k to 6k.
Back when this general practice started it was true that there were more buyers than property and to not put down a reservation fee would certainly risk the property being sold to another individual as everyone stampeded to buying their dream home or investment in Spain.