No website can guarantee that your account will never get banned. But as long as you keep the following three tips in mind, you will not get banned.
1. NEVER give your account password to anyone. Gold sellers do NOT need it. They only need to know your character name (for PS4 also needs PSN).
2. Choose a reliable and respectable online PoE Currency seller like MmoGah, which can provide you safe PoE currency but also a pleasant shopping experience.
3. Your chances of being banned are probably less because GGG might be willing to turn a blind eye. After all, you're buying currencies, which, in turn, means you'll play longer. And proved by facts, that there have been no players get banned for buying legit PoE currency.