Military Starbases are not military force in their area of influence. What they do is 1) defend themselves and 2) enhance starships within their area of influence.

Whenever you upgrade a starbase you will see options that affect either the starbase itself, or friendly fleets in the area of influence. Basically, there will be a little starship icon next to enhancements that benefit fleets (these don't benefit the starbase), and others only affect the starbase's base stat line.

It's generally much better to enhance your friendly fleets than it is to enhance the starbase. The computer AI will attack starbases smartly, and whatever you put in will usually be insufficient to stop an attacking fleet (fleets attack with combined stat lines, whereas a starbase is basically just a single ship on its own).

Therefore, the smart way to use military starbases is to put them in areas where they can affect multiple friendly planets (this is true of economic starbases especially), and to keep fleets fighting defensively in these areas of influence.

I'm confused about what you mean with "ships to sell" - as in - to the computer? I've never practiced that. If you have sufficient peace to sell ships to the enemy AI, you're probably playing on a level of difficulty that's too easy for you.

Good luck out there.

Or, you could go to the diplomacy screen for the alien of your choice, select one of your ships, and see how many bc's the alien will give you for it. First, click on the ship you want to sell (in your column), and then, in the alien's column, click on the "bc" number on the top. When the slider screen pops up, adjust the number up and down until you get the highest number with the words in the dialog green rather than red. That means they will accept the deal. I've never tried to sell a ship, It would be interesting to see how much could be obtained that way. It might cost you more to make it then you would get by selling it. Good luck!