Go on deactivated and click ''edit'' on the item you wish to put on the market. Once you're in the edit page, (for a head) you will see an upload images box, below that should be a box with a number already in.
Change the number to the desired price you want to sell the item for and press ''Update''.
Your item will then be put on the market, and can be viewed by pressing ''shop'' and looking at ''market inventory''