It all depends on your luck and who you sell to. I had a high level account that had no history of RWT and it got banned within hours after selling the first time.
There's no way to "reduce" bans despite what people will tell you. Trading items looks more suspicious, it's just luck and how much you trade at the end of the day.
Jagex aren't an indie company who aren't incapable of tracking an account's activity, seeing someone suddenly trade their bank over to a level 3 is an instant give-away. Sometimes you slip away, sometimes Jagex will catch you with detection, other times someone will tweet you to Mod Weath.
If you value the account more than the gold, then don't sell gold.
Buying on the other hand is safe.