Well you don't buy "a luggage," you buy "a suitcase," and the adjectives are for a singular suitcase and not plural luggage, so i don't know... suitcase makes more sense to me.
I agree - stores don't sell luggage, they sell suitcases.
Luggage is a collective noun for everything - all your suitcases and bags and everything in them.
"Kuffert" is similar to "cofre" in Spanish, which means chest/trunk. Interesting :D