Can I put in a recommendation for
www.seat61.com - a site with details on getting trains to pretty much anywhere in the world from London (he doesn't sell tickets, but it's useful for planning as he gives timetables, prices, route advice, links to providers)
We travelled to Greece by train and ferry last year and managed to buy French and Italian tickets from their own online services (voyages-sncf.com and trenitalia.it). SNCF will post tickets to you and Trenitalia do e-tickets. SNCF site has a timetable query engine available in English. Cross-border tickets can be a challenge or lead to very poor fares - we ended up buying seperate tickets for Marseille to Nice and Nice to Genoa because there were actually discounted fares available for that.
(sorry for the essay!)