I think the point you’re missing is that the normal course of business for most products is that many distributors compete to sell the same item on Amazon. Go look for example at Canon cameras, you will see 10-15 different legitimate sellers all offering different prices, all on the same detail page - you have to click through to “this item available from xxx sellers”

If you’re interviewing sellers, what do you ask them in the interview? “Are you selling counterfeits?” What are your grounds for allowing them to list an item?

If you want to inspect a new listing, what do you want to inspect? Maybe you inspect the first shipment. Fine. People will figure that out and send a small first shipment of real items, then start sending fakes.