I'm asking for myself, and for a friend from a past job: do you know of any place to get the Loeb edition of various specific works (say, Abe Books or Half-Price-Books, or some universities working on digitizing their collections?) at non-exhorbitant cost?
Cheers!
As far as finding good used copies online, I haven't done that and can't say which of the sellers out there is the best to deal with.
The translation of Thucydides by Benjamin Jowett is also available free online.
The Penguin translation by Rex Warner is very popular. The Penguin translations tend to be more readable but sometimes a little freer in how they render the original. There is a fairly new translation by Martin Hammond in the Oxford World's Classics series. Amother recent English translation which has gotten high marks is by Steven Lattimore. He is the son of Richmond Lattimore who published a number of translations, including a very good verse translation of the Iliad. Richmond was the younger brother of Owen Lattimore.
The Loebs tend to be more literal--the main advantage to having them is if you have some knowledge of the original language (Greek in this case, Latin in others) and can look over to see what the original wording was. Otherwise to buy the four volumes is a lot more expensive than buying one of the paperback translations.