The verdict
If you mainly sell downloads, courses or subscriptions, and traffic comes from your own channels, I'd pick Sellfy. File delivery, download limits and upsells are what it is for. If you need a credible company site first, with pages, a blog, bookings and a few things to buy, I'd pick Squarespace. For most small B2B teams the website is the bigger job, so Squarespace is the safer default. If you want both, you can embed Sellfy buy buttons on any site, which keeps product delivery out of your site builder. Check how leads reach your CRM before you commit to either.