I believe this is due to the platform you're trading on. Certain websites exchange RuneScape Gold with players like garbage collectors, however when they sell them to players at a different cost it is not acceptable for me. Therefore, you must choose a legitimate platform or else you'll be suffering quite a bit.
I typically trade with RSGold on the RSorder.com website, in part due to the fact that no matter what it is it is priced open, fair and in line with the market prices. Also, they have a great commitment to service. When I sold just OSRS Gold at RSorder, the customer support was similarly positive, and they would like to get OSRS Gold in the hands of players. This is because, considering the security of accounts, RSorder only trade items that are purchased online by real players as well as OSRS Gold. This means that players can be at ease purchasing cheapest osrs gold.