I worked for largest eCommerce employer twice and have built scraping and search systems at scale. I know a thing or two in eCommerce shopping sites. Do you know how much infra and people it takes to keep pricing, product, variant data up-to-date? Hint: It's significant capex investment.
Problem is entry point funnel in the shopping journey. If all he becomes an aggregator, it is no different than thousands out there. There is Shop.app which has all access to seller data and analytics. As I said, this is not a search problem. It is product discovery problem which you also seem to agree with. Unfortunately, not a lot of people/teams have managed to solve dicovery problem when site itself is fairly unknown.
Michael from YC also talks about product discovery being a tarpit idea.
I am not saying his search won't work. Search in itself is a solved problem here. You enter criteria/product name and you get results. But, unless you truly solve discovery, which in my opinion he can't do due to lack of first-party data, it won't do much besides a hopper site.