For purchasing ferry tickets in Greece, I cannot recommend Danae Travel highly enough. I ordinarily prefer to purchase tickets directly from the travel operator, but for one ferry journey in Greece there was no such option, so I used Danae. What a lucky break.
The service was incredible. The prices on the easy-to-use Danae site were the same or lower than those posted by the ferry companies themselves. All fees and terms were clearly and repeatedly posted on the web site (contrary to an earlier post on this site about ticket pick-up fees).
Then, the Danae staff continually kept in touch with me by e-mail, advising me of complications due to possible strikes. Chryssa in particular was fantastic, responding within minutes to my questions, and in fluent English. Most incredibly, when a family member became ill on one leg of the trip and we had to cancel tickets we had already purchased, Chryssa responded immediately with clear directions and arranged a refund. I have never seen better service from a travel agency, and now could not imagine buying ferry tickets in Greece other than from Danae.