Shopify

• Admin is in English only
• No support for SEPA/ELV payments

Big Cartel

• Beautifully simple
• Interface entirely in English
• PayPal payments only

Supr

• Based on WordPress, which is probably the reason you can’t customize the page html itself; customizations are CSS-only.
• Right amount of complexity.

Versa Commerce

• UI a bit antiquated
• At times I find it even confusing.

Oxid

• Can probably do anything, but is zero fun.

Goodsie

• No German interface, no ELV.
• Pretty cool UI designer though.

3dCart

• No German interface, no ELV.

Shopware

• On the upper end of desired complexity, but still within acceptable bounds.
• Setting up payment methods by referencing a PHP file in the UI? Ugh.

Amazon Webstore

• Insane amount of verification bureaucracy to even get an account.
• Interesting UI designer, but tiresome to use, and limited in what it can ultimately do (fixed header, footer).