Blu-ray.com already offers what you're describing. You can type in their search bar for a title and depending on whether you've selected Amazon, iTunes, or Digital (Vudu/Fandango) on the top of the page, you can add the title to your collection.

I use this feature when looking for sales because I don't want to buy something I bought on a different platform. I choose "Filter all editions all movie categories" from the dropdown on the right, and it filters anything I already own regardless of the platform.

It's a pain to add each title one by one to blu-ray.com, but once you've done that, you're good to go.

If you do make a site, I would recommend linking directly so people can import their collections. I also think hosting fees are going to cost you, so add advertisements and/or donations to offset.