I believe this is due to the platform you're trading on. Certain websites deal osrs gp sale with players like garbage collectors, however when they sell them to players at a different cost that isn't acceptable for me. Therefore, you must choose a legitimate platform in the event that you do, you'll suffer quite a bit.
I typically trade with RSGold on the RSorder.com site, mostly due to the fact that no matter what it is it is priced fair and transparent, and in line with the market prices. Also, they have a great friendly and helpful customer service. When I sold just OSRS Gold at RSorder, the customer support was similarly enthusiasticand they have would like to get OSRS Gold in the hands of players. Because, in consideration of the security of your account, RSorder only trade items that are purchased online by real players, and OSRS Gold. This means that players will be at ease buying OSRS Gold and runescape powerleveling.