My two cents worth, I am in business here in the UK, don't knock me yet, not an Estate Agency, but my Daughter has one, sales and lettings in a reasonable affluent area, and coastal. Not Spain granted. And thats the secret be in an area thats not run down or scruffy.
She does the internet of course due to the many lazy gits who prefer to not get up and visit her agency, the internet is another tool in the box you will need 100%, she opens any hour she can, which I believe is good, she has sold house's when other agents have shut up, like she says you have to be their when people are about because you never know.
This Daughter has, I don't remember except quite high what qualifications, but for lettings and sales that go wrong she can stand up in court and represent the owners or landlords in a legal capacity, perhaps not needed in Spain.
You will need a shop window which is your eyes to sell your goods, when she started she rented an empty shop and for 6 / 8 weeks had a large advert in the window telling what the shop would be and if you want to sell or rent your house out...Come on in. It worked, first day and not really open think she said she got about 5 people to sell house's and one to buy, which of course she had not much to sell...Yet.
It progressed slowly upwards from then on, often people come in and mention they saw her ad on the internet, so yes you will need that in the modern way of doing things.
Understandably the UK is not Spain, but the principals are the same, you are selling / advertising an item that hopefully someone wants to buy, her outgoings are rent, rates,wages etc, no actual stock to buy. Its fair to say the first year or two was hard, people generally don't trust estate agents, I told her to be more then honest, speak your mind, leave nothing to chance, treat everyone no matter who with respect, even the rubbish people who will give you a hard time, let no one beat you. Remember it could take a good few weeks to get your fee's coming in so having a back up amount is a must.
She has a very good solicitor who does a lot of her legal part of the sales but she lets the clients make up their own minds about that, some use him so don't.
You will find it hard, but has anyone told you life is easy? Hope something I said helps in a very small way, I realise not so much to do with your original question.
And good luck.....You won't know if it fails unless you do it.