In my experience, you couldn’t just setup an account and start selling, you had to contact their sales team and they let you know if they want your business.
Last I checked, Polar was using Stripe under the hood.
EDIT: Yup. Via their pricing page:
Polar is currently built on Stripe, and we cover their 2.9% + 30¢ fee from ours. However, they impose a few additional fees for certain transactions that we need to pass on.
Polar.sh is another one.
The thing you're talking about with Paddle is just checking you're not selling offline, physical stuff.