Facebook marketplace has an shortcut in the bottom of your timeline, if you can’t find it you can open Facebook, click on the three lines icon , then choose the icon of marketplace from All shortcuts, if this didn’t help you, probably you don’t have the icon because of some reasons, such as you are under 18, as this feature is only available for adults, you can use the marketplace of an adult in your family, another reason is that you are in a country that doesn’t support the marketplace, simply you can change the country from your settings, also you may have a new account and the icon doesn’t appear yet, just wait few days until the icon of marketplace appears, and the last reason is that you have an unsupported device, in this case you can use another platform to access the marketplace.