Why should I use a real estate broker?

You would be well advised to use the services of a reputable agent. (Who is reputable? Ask around…Skiathos is a small island and it will become very quickly apparent who can be trusted and who not).
An agent is there to protect your rights, to help make an agreement between purchaser and vendor (often involving translation as many Greeks do not speak another language and most purchaser’s Greek is not sufficient) and smooth the bureaucratic process. If he or she is good at his/her job, he will have earned his commission many times over.