Selling is an action which can be performed on creatures and most items. In the case of creatures, it will list it on the Market Place. In the case of items, the item will get deleted and you will get an amount of silver in return.
Selling a creature is done through the creature's Manage page. One of the three links at the bottom is 'Sell' followed by the creature's code or name. Clicking this will allow the player to enter a price (in Silver) or cancel the trade.
Once created, the sale can be managed through the Market Place in the Bazaar, where there is a link at the top to view the creatures the player is selling.
Items are sold through the player's Inventory. Not all items can be sold; those that can will have a box to enter a number and a 'Sell' button. Selling items is rewarded with a certain amount of silver.
Once sold, an item cannot be recovered.
There are items on the site which are there purely to be sold; these are called Junk items.