My for display only

Is there a question?

I think that means the set is made of a material determined to be unsuitable or unfit for use. It's a set for display only, not for actual use in serving/drinking tea. The porcelain could be porous, and materials or chemicals used in the process can leech out into your tea, causing health concerns (poisonous results).

Or the material may just not be fired for holding hot tea (a cheaply-made repro of the real, more expensive set, one that won't cost much if it is broken accidentally by customers handling it), and the cup could just fall to pretty sludge after absorbing enough hot tea...

The set is just for window show or advertisement, not for actual use. I wouldn't even put them up in my cabinet for show, as someone will pick them up one day and try to use them.