It is tough to find products which are on the clearance tables marked down enough to pay Amazon’s fees and leave you a profit it you make a sale.
Many of them are not in NEW condition, even though they may be unused, and more of them will be products the store has discontinued because they do not sell well. Other will be old packaging or manufacturer discontinued items
The manufacturers of those products are under pressure from their authorized resellers to protect them against online sales at prices which are too low, or from unauthorized sources.
There is always pressure on manufacturers to act against unauthorized sellers and more and more are acting.
Amazon reacts to manufacturer complaints, and it reacts to buyer complaints. If what you sell is in old packaging, or appears not to be the latest and greatest, you can get complaints which translate into inauthentic. And if you buy at retail, you have no proof of authenticity.
Many, many Amazon sellers have sourced at retail, and that includes me, and many no longer do so because the time has passed when you can safely make money here doing so.
There is no level of expertise that can keep you from eventually running into problems. New obstacles occur daily, and Amazon reaction is usually to shut down a seller.
When I lived in Chicago in the late 1960’s, the police strategy on the street was “everybody goes when the wagon comes, sort it out at the station”. That’s the way it is here for many types of merchandise bought at retail. Amazon closes the listing, or suspends the seller or both.