You have an online store. You want to sell more on it. (If you don’t have an online store, or if, for some reason, you want to sell less, this isn’t the post you’re looking for. Move along, please.) There are two main ways to do this: get more traffic, or increase your conversion rate (i.e. sell to more of the people who are already coming to your site).
Although the trustmark is free for (only) up to 500 visitors per month, it’s still worth to try.