Yes. You could see it as a reverse of either of those sites. And you also identified the main problem I've run into. My thinking is that this service would maintain a "snapshot" of your pantry by several mechanisms: 1) auto-add ingredients to it by scanning your receipts, 2) auto-update / remove ingredients as you use the service, 3) manual curation.

Ideally, there would be very little manual curation, but I think it might prove to be challenging if users decide to make something without using the service. Plus, there is the initial setup that could be a turn-off.