-(으)ㄴ 거 makes "the one that is X," and 말고 rules it out so you can point at what you actually mean: 많은 거 말고 뜯어 먹을 거 좀 사, "not a lot, just something to munch on"; 이게 더러운 거 말고 깨끗한 게 정상인 거잖아요. Every line here uses the 말고 form, and it always names the rejected option before the wanted one. It is ordinary spoken Korean — the quickest way to correct someone mid-sentence.