The verdict
If your goal is to get a page live today and see whether anyone cares, pick Carrd. It has a free plan with up to three sites, so the test costs you almost nothing.
If you already know people will pay and you have a product to hand over, pick Sellfy. Checkout, secure file delivery and subscriptions are what you are paying for, and building them yourself on a one-page site is a poor use of time.
If you are unsure, I'd start with Carrd to prove demand and move to Sellfy once the first sales are coming in. The two also sit together without trouble, as Sellfy lets you embed buy buttons on other pages. Check that your Carrd plan allows the embed before you plan around it.
Neither suits a company that needs a multi-page site with a blog. Look at a full website builder for that.