If you are not here for much of the year or will be absent during the building of your villa or house, you would be advised to have someone in Skiathos who can look after your interests, keep an eye on construction work and pay bills as necessary.
If you cannot be in Skiathos for the signing of the contract of sale, you will need to give a Power of Attorney to someone here to act for you. This can be your lawyer, the real estate agent or someone resident here who has your trust.
This is a fairly simple document and can usually be drawn up by the notary within 24 hours.