At the market you can sell or buy nearly every thing. Just some Traders ask after the origin or purpose for the good. If you`re looking long enough, then there is a possibility to find forbidden or occult items. More then one creature of the night use the Market of the mortals to receive equipment or to sell it. 
The Market Place is where you can sell your items to other players as well as buy from them. Since here the buyer get lower prices than the Merchant's and the seller earn back more coins than reselling to the Merchant, both are benefited
Buying[]
If you have the necessary number of coins the seller required, simply click 
Selling[]
You can only sell unequipped items originally bought from the Merchant. Items bought from other players through the Market Place are Sound Bound and can only be resold to the Merchant.
To sell an item you need to type your desired price (at least its resale price and at most its sale price at the Merchant) and select the duration you want that item to be on sale. You have to pay a small fee for the item to be displayed (2%, 3% and 4% of its price with the duration of 2h, 8h and 24h respectively). When this duration ended and the item is not bought, it will automatically return to your inventory.