When I bought the Centurion I hat not enough credits for a rocket launcher. I thought I take two torpedo launchers and replace one by a rocket launcher later. Now after many, many fights I see this is not possible. Uuhuuuhuuhuuuuuuh, I have to play it all again, nononononoooo! :-( Please, please, please make launchers sellable, because this is an arbitrary limitation.
Then I bought a tractor beam and slots 5 and 6 were offered for mount - that were already occupied with the torpedo launchers but I could mount it anyway. In flight the Gun's menu only showed the tractor beam which could be normally used. After landing I still had the torpedo launchers.
I could even buy a second tractor beam and mount it to slot 6, funny, isn't it? In flight both tractor beams worked together.
After landing again I sold both tractor beams. The torpedo launchers were still present, they were offered for selling but not sellable. Then in flight both torpedo launchers were not displayed in the Weapons menu. I could not use them or sell them or buy new ones.
I think the "unsellability" of launchers is corrupting the logic.