Travel Agents - Booking Offices..
Over-sea travel, as against 'overseas' in Greece is an everyday event.  There are so many islands that scheduled ferries and aircraft run almost like the bus services on land.  Rail services, both local and international, are possible only on the mainland.  Bus services from some islands, including Crete, actually run to Athens, catching a ferry en-route.  It isn't much of a surprise then to discover that there are many purely ticket offices, maybe as many as there are full blown travel agents.  Shipping and airlines often have their own booking offices too, although they will, like all the others offer help and advice about their destinations.  Be aware though, a travel agent can sometimes offer a discount that the carrier doesn't offer!
ANEK Lines central office, Boat Tickets, Kastelli, Kissamos, Nomos Chanion, Crete.
The major bus company on Crete is KTEL....
KTEL is the national bus company of Greece and has services almost everywhere anyone might want to go.  I can get from my local bus stop in Kolimbari to almost everywhere else in Greece at amazingly good prices within a day.  Although from Kolimbari my day will have to start round about early evening, changing at Chania, because the bus to Athens will have to catch the evening ferry to Piraeus (Athens seaport).  Not many holiday-makers might want to go that far, certainly not on a short holiday. 
But certainly you can tour Crete by bus staying overnight in different places in the certain knowledge that your transport will be there next day.  Any KTEL office will supply tickets; book seats and supply schedules.  Be aware though, that although the majority of the buses are in fact Pullman coaches, complete with air-conditioning; comfortable seats and all mod-cons, some, particularly those traveling to the remoter destinations, may not be!  It is best to book a seat if your journey is long or you need to sit, but don't be surprised to find your seat occupied or afraid to reclaim it.  Locals will not know the seat is booked unless you tell them.  Or ask the conductor to assist.  Remember that the conductors will load your luggage if you have some and return it at your destination.
KTEL buses, Leoforia, travel everywhere. Crete, and to mainland Greece.

On major routes in and between tourist areas in season the crews are happy to deal with foreign visitors and usually make sure you don't miss your stop.  But beware, bus inspectors are both helpful and frequent, but will want to see your ticket...

Travel Agents are usually excellent...
Of the local agents here we have used  Canea Travel, whom we use fairly regularly;  Dorkas Travel; and Kiriakakis travel in Chania.  A good agent is essential as problems can and  do occur.  We once booked return airline tickets by 'phone, from Iraklion to Gatwick UK, only to find when we went to collect them that the airline had changed both the times and the arrival airport to Stanstead, and that there were no alternatives available.  Our travel agent was able to tell us immediately that the distance from Stanstead to our destination address in London was only a couple of miles different than from Gatwick.  In the event we discovered that we liked Stanstead far more than Gatwick!  On another occasion Canea Travel arranged free overnight accommodation in Chania for our niece; her husband and three young boys when the airline they had booked them with 'bumped' them at the airport and they refused to split up onto different carriers.  A good travel agent not only sells you a ticket, they sell you travel and make sure you are taken care of.
Meet Stella - "You are not really putting me on the internet are you?!"  Stella works at Canea Travel Bureau and makes sure you get first rate service.  Travel agents, Chania Travel, Stella speaks perfect English.
From outside Crete, say from the UK, you may find it somewhat difficult to find accurate itineries and online bookings for travel without a good travel agent.  When we first came over here to live we came by car.  This is a good way to come if you like driving and independent travel.  But you may find booking tickets for Greek ferries from England difficult.  Even though the AA were good at getting us to Italy they were unable to book either us or our car from Ancona (Italy) to Greece and ultimately to Crete, describing the shipping lines as 'Domestic'.  Meaning that we would have to book these tickets from Ancona ourselves.  We discovered Viamara Travel in London.  They had no difficulty booking our tickets all the way to Chania.  A member of the staff there and her husband actually traveled our intended route and knew it well.  She gave me (absolutely accurate) directions and tips which turned out to be better than the computerised  route plan we had obtained.  We repeated the exercise with friends a couple of years ago, again getting our tickets from Viamara and without really needing a map! 

We could of course simply booked the tickets when we arrived in Italy, but at certain times of the year ferries become fully booked and arriving to such a situation might mean extra overnight stops and extra costs.  Let you travel agent do the walking - they can save a good deal of hassle, time and money.  But choose a good one.

Travel by Internet..
The ultimate Greek travel agent website here on Crete has to be Paleologos Travel of Iraklion.  At their website, direct from your computer screen, you can learn almost anything and everything about traveling to or within Greece.  Online booking is a feature.  I sometimes think that if I asked, they could probably tell me the name of captains and crew of the aircraft and ships, to say nothing of the drivers of the trains and coaches running on any particular date I cared to mention.  If you are reading this page you obviously have internet access.  Have a look at the Paleologos sites listed on our home page or the website directory page herein.