You can't sell locked mods either. So I lock mods with great secondaries or I plan to slice so that I accidentally sell them
I think there needs to be a mod flag feature. Where you can flag mods red/blue/green etc.
Since you cannot sell mods from the main menu, I'd like to flag some mods when swapping between two characters (since that's not an option) or when I level a mod to 15 and get lousy secondary boosts and want to sell it right there and then, a flag option would be nice.