as i understand you just compare two things in one menu. well, in one instance of the menu. f.e. you open your inventory and want to compare two weapons, you push C on one of them and then on the other and two side by side windows open. Not very convenient, but it seems its because you can use up to 4 active weapons so it need to know which one to compare.
Though you can't do that while buying things in the shop. You only have either sell window or buy window. And in the sell window you can't see whats equipped. So for now the only way to compare items is just looking at your inventory on equipped items, and memorize it, lol) Maybe i'm blind, but i don't see another way to do it.
It would be cool to see some small windows in buying\selling UI with your curently equipped items so you can compare it with C button.
PS sorry for not-my-native english