Hi, as far as I can tell the Scirra.com store is still up. I'm unsure if it let's you complete a purchase though which I feel might be your issue. https://www.scirra.com/store If this doesn't work, then yes, the store is likely down and moving to the new Construct.net domain. Many of the features on Scirra have been buggy and going missing for years due to them migrating the site.
Hope this helps!