"Ubrania" is a standard word. But you're right "ciuchy" is a quite common colloquial word.

You can always omit "oni/one" in polish sentence. It is less common than omitting ja/ty/my/wy.

Selling "odzież" sounds more or less okayish to me. But buying? I don't like it. It sounds as if it really needed some adjective. Mostly "odzież używana" (second-hand clothes).

But technically... ok, added.