Version 7.1.0 will introduce a tool to let you open multiple links to CardMarket or TcgPlayer more easily.
If you have more than one card selected you can enable the Shopping Assistant through the context menu:
Or via the "..." menu:
This will open a tiny tool that will reduce the number of clicks required to open all the links:
You can now simply click on the "Open in a new tab" button, hit CTRL+SHIFT+TAB to get back to your main tab (this is because browsers will not let JavaScript open a tab in background). Rince and repeat :)