If you’ve had your Facebook pixel installed on your site, you should be able to retarget these people. To create a custom audience for retargeting, go to your business settings, click on Audience, then click on create custom audience, choose your website as the source, when you hover over the options, choose “include” people who have purchased from your site - these events should be set properly; the add to cart, initiated purchase, initiated checkout etc. After creating this audience, use them as your audience on your ad sets.