If it is your first time selling i would recommend selling a smaller amount first. Websites I like to use are Ezrsgold and Rsgold420. For that reason I choose to sell to reliable websites that have 24/7 staff and good reviews on trustpilot. This happened to me and this happened to a lot of players. People in the forums, discord and facebook are usually looking for people who are selling for the first time in hopes to take advantage of them and take all their gold.
Selling Runescape gold can be tricky, because there are a lot of places out to get you.
Here's a quick step by step guide that will help you do so! Always wondered how to turn your OSRS or Rs3 gold in to real money?