It is automatically offered when you build the starbase, but there has to be some resource near it for it to give you that option. At least that is how it appeared when I had it happen to me. Not sure what the range is though. Would be nice to know.
UglyElmo
I had sent 4 diplomats to each of those factions earlier in the game and never refused a trade. They actually were gifting me 175 credits through most of the game. Was hard keeping factions liking me when there were wars, but judicious use of gifts and diplomats did the trick.
Has anybody had two separate civilizations gift you all of their remaining planets/ships/star bases when they retired from the game, when being conquered by other civs? They said I treated them so nicely, they were giving me everything so I could get revenge for them. I had a power ranking of 6th out of 8 civs, and was way behind the first and second ranked civs, until this occurred. It brought me up to 3rd place, slightly behind the top two, and I was able to quickly get a