1. Когда какой-нибудь клиент надевает какое-нибудь украшение и говорит: "Не смотрится". Что это значит?

Something small (when you hang something on, for example) is always - надевать; for any object - your or other person.
It means that it doesn't fit/suit this person or his dress, etc.

You can use "один" that way: "Один из клиентов" or "Один клиент сделал <something>, а второй - <something else>".

2. Когда какой-нибудь клиент смотрит на витрину, которая выключает ювелирные изделия и говорит: "Проба?".

На витрине <- place. Means you are there and watching. You are staying on the showcase and watching somewhere.
На витрину <- direction. Means you are watching that direction (direction of that thing). You are staying somewhere and watching at the showcase.

Выключает = turning off. May be you meant that it included jewelry? In Russian we always think that things are on the window.
You can say: "на которой лежат ювелирные украшения."

Yes, I know that jewelry are usually inside a glass box. If you want to say inside you should use other word for the showcase.

Don't know what they mean. May be some slang for jewelry. Never bought it.

3. Как cказать по-русски: "We spend a lot of money in the advertisment and promotion of our products and services"

Мы тратим много денег на рекламу и продвижение нашей продукции (наших товаров) и услуг.