and you ask about 'buy gold' google says: Mǎi huángjīn ( 买黄金) which would be that 黄金 one.
would 买金 make perfect sense too? probably...most used in china..idk.
when I go searching for best, I check all these places, forward and back. I search in google for websites and look at the highlights, to see which variation is more frequent.
There is also traditional Chinese (hong kong) and simplified (main land). 金 is the exact same character between them. But, 黄 is slightly different. (黃 = hong kong) These 2 are so close that both areas will probably understand each others visually.
I find in domain, and especially chinese domain, short form is best. if you can convey exacting message in fewest character, do it.