i think that some factors that may arise the banning risk are:
-If the gold that you're willing to sell comes from popular goldfarming methods AND it's your main activity in the account. Since i think it's a obvious behavior that may trigger the RWT ban system.
-the gold trading activity of the accounts involved in gold swap. For example, if you buy or sell to somebody that have been using his account too many times recently for trading gold (popular buyer/sellers), instead if you trade with a legit player that barely do a gold trade, then i think it could be safer
About the quantity. I actually couldn't say a specific amount, i think there's a risk no matters the amount traded (unless it be so pretty low amounts like less than 5 m )