The problem arises from the specific nature of armor upgrades , there is no armor count like in... well , any other FPS in existance

NS features armor that can be welded , and alien carapace that has to be healed (like hit points) , maybe Tremulous could find some inspiration here. Armor that can be repaired by a construction kit and carapace that requires the overmind (or a booster) to heal would have some significance in gameplay. Teamwork would become crucial at once for humans , making battlesuit rambos short lived (as they are in NS)
Selling damaged armor would only return a fraction of its price , of course
