Mayocide when

This already exists, albeit not in federated form. It fundamentally doesn’t work because the market players have an incentive to withhold as much information as possible, because any mistakes consumers make from not comparing prices is direct profit surplus.

Collecting the information in the first place is also difficult, because it would essentially require getting the consent of most sellers (which they are disincentivised to provide), or just scraping it (often illegally).

Thus, such an aggregator requires too much work/risk, which needs to be compensated for. Consumers generally don’t like the idea of simply paying for independent advice/brokers, so we are stuck paying in other ways, such as via personal data and behavioural surplus for commercial tech sites, of which numerous exist.

Most search engines such as Google (eww I know) already have a shopping specific search page. eBay and Gumtree also have existed for decades.

eCommerce platforms like AliExpress and Amazon also already do this, if you set the filters to only be third party sellers.

There’s also category specific aggregators such as PCPartPicker/Newegg.