"Why does he buy a black shirt?"
Translation:Kenapa dia beli kemeja hitam?
In this context, is there any difference between "beli" and "membeli"?
The me- prefix is dropped here, just as it's done in normal speech.
'Kenapa' is used here (instead of the more formal 'mengapa').
Kenapa dia beli kemeja hitam ?
Mengapa dia membeli kemeja hitam ?
Yes, it's perfectly okay like this.