Many shopping sites are based on woocommerce, which is an ugly hack transforming a blogging platform in a store.
Like if you take a school and made it a supermarket with all the goods scattered on the desks in the classrooms.
Sucks at performance and sucks at search.
The actual reselling is done by a shell company that disappears and reappears with a different name when hit by lawyers.
This website just gets referral money from each sale, they’re not selling the grey market keys