To get to the market, type ::market in-game and you should appear in Falador Gardens.
How to Sell items at ::market
To sell items at ::market you are going to need to setup an Advertisement. *Note that ::advert only works in the market
To setup an advertisment: Type ::advert (What you would like to advertise you are selling) and it will auto type it for you. Most people selling will be AFK, and saying "Check my shop"
How to view other shops and your own shop
To check someones shop, simply right click there name and click "View Shop". To see (add or remove items aswell) from your own shop, do ::myshop and it will open. Take note that you can only add 10 of each item at a time. (Some items must be noted to sell.)
Always check the price of the item you buy
There is a known "Scam" going around if you'd like to call it that. They have stated that the items for sale are 800k each, but in reality they only have 1 slot listed for 800k each (hoping you click to check the price and see 800k) and the rest are sold for 8M each. ALWAYS check the prices! Don't be fooled, theres nothing worse than being fooled.
I didn't recieve my money after my item was sold!
The money doesn't go directly to your inventory. To obtain your money from selling an item out of your shop use the ::Collect command and then you will recieve your hard earned money.