Generally speaking, a game doesn't give you options without valid scenarios in which to use those options. Specifically, there's probably a reason we're given the option to sell towers in Squadron TD, but I never sell them because I don't wish to waste what I've spent.
After wave 20, is, for instance, a Dark Mage worth keeping? Or is 50% of his paltry value worth putting towards yet more Lords of Death? Heck, I'm not even sure Dark Mages are worth building in the first place, considering how effective Fire Archers are.
The forums for Squadron TD are full of build orders and opinions, but low on detailed info, or advice for playing beyond wave 10.