There are a couple ways to get to the market page. You can find a link on the actual city page, or when in a city, you can use the Local menu on the navbar. You can also narrow your search, by going to the shop page, and clicking on the market tab there. Here you can buy items that other people are selling. To sell your own item, you must first buy a Stall in the City Hall. From there, click on your Business button, now available in that city.
On your display section, choose which item you have, in your Inventory and select it. Pick how much you want to charge for the item, and click offer. You can check your Professions page to see if the item sold.