In every town, and sometimes in your travel, you can come across merchants. talking to them about conducting business will open a sales menu. to sell stuff, go into your inventory and click on items you want to sale, this will start the sale and add show how much money you will make, and how much the other guy has. you can buy stuff from him if you want at this point, the money you would be making from your sells will be deducted when your doing so.
Once you are done, just accept the transaction and the money and items will change hands.