Everything should be for sale, at a rational price.
Russia absolutely would have sold the USA Siberia, just not for the same price/acre as they did Alaska. Heck, Mexico sold us the Gadsdend Purchase - the small strip of southern Arizona/New Mexico. That was a part of the core Mexican territory. Yet it was happily sold to the USA, just at a price/acre that was 40 times what the USA paid for Alaska.
The key here is to get the AI to recognize that most of a thing's worth (ship, fleet, technology, planet, starbase) has very little to do with the specifications of the thing in question, and much more with the thing's UTILITY. How the AI manages to compute utility a hard problem, since humans don't do it very well at all, either.
Clearly, a single starbase far-distant from anything else a player owns is of fairly high utility, compared to one within 10 hexes of another. But also, a fully-stocked military starbase next to a potential conflict zone is worth many, many more times than a single starbase way out in the middle of nowhere, even if that military starbase is right next to several others. There's a whole lot of math and judgement that has to go into assessing Utility, and it's not at all clear how to make this kind of assessment.
Not to mention that Utility is judged by not only it's usefulness to the seller, but also to the buyer.