I would strongly suggest NOT using Crystal Rentals Spain for any business at all.
Although I'm on the renters side, they were totally unacceptable as a rental agency and we even have emails between the house owner and the agency about the poor relationship they have. They do not treat their renters well at all, in fact they offer no service whatsoever AND they arbitrarily overcharge with extra nights and they don't refund the overpayments OR the deposits.
They are definitely part of some larger scam and they will take your money and let your property fall apart and probably never give you the money, if they do actually rent it out. Paul and Jenny are not running a legitimate business.
And don't believe that they will return your 500 euros if not rented! Don't believe a word they say.