There is obviously no way to be 100% safe, if you want to be safe you have to go in nosmall and buy nds.
If you buy gold from third services you will always have a chance to get banned even tho it is significantly lower than when you buy a whole account (especially if the account is well-known like friends with GMs or well-known from other players).
Btw when you buy gold on official, the ban rate is far from being 100%, I don't really know from where you got that number but they barely ban people buying gold (I guess if you do it for an insane amount of gold like couple billions or too much time or if the gold you bought got botted, you will end up banned)