Why is 'pembeli gedung itu adalah kaya' wrong?
When "is" is followed by an adjective, don't translate it to "adalah" in the Indonesian sentence.
I am rich = Saya kaya (not Saya adalah kaya)
When do i and when do o not try to represent 'the' in these sentences? Pembelinya gedung itu kaya is wrong because...
You only get to use "itu" to represent "the" one time in Indonesian, to refer to the whole phrase, and it goes at the end of the nominal phrase. "Pembeli gedung itu". That's it.