Heirlooms in The Sims Mobile are special figures that can be acquired in the game. They unlock new features or hobbies. You can earn these souvenirs by taking out your active Sim. You have to note that taking out an unplayable Sim won't get you any Heirlooms. You will earn them only by sending a Sim for retirement or by sending it to another player.
You can sell your Heirlooms After the latest update you can only sell duplicated Heirlooms - it is required to have at least two of them. You must sell a copy of the Heirloom. This is difficult because there are little chances that a copy will be drawn for you. Additionally, you cannot buy one either. You will receive coupons for selling Heirlooms. You can spend these coupons on other types of Heirlooms.