There are a number of different ways to sell items. This will touch on a few.
If you are at a market you can 'hawk item' to try to find a buyer. You can then 'haggle'. When you feel like you've got a good deal you can type 'deal' to do the transaction.
If you want to sell an item to an NPC, you hand them the item. They will either quote you a price for it and you can respond by speaking directly to them, or they will simply pay you for the item depending on the situation.
