Trustworthy but don't buy through digital currency.
Ariba Gold is a reliable service that I have used and can confirm they are legitimate. I would advise to stray away from purchasing gold through digital currency as it can take an extended amount of time for their accountants to track the purchase in order to complete the exchange. First time it took them around an hour and second time took about 36 hours. Either way they will be able to find it I just had to go through about 8 different representatives from Ariba to finally find someone who can track digital currency transactions.