What platform should you use if you want to sell physical products that are not print on demand?

Amazon Seller Central is the correct platform because it allows businesses and individuals to sell physical products directly to Amazon customers. You manage your inventory, listings, pricing, and fulfillment (either by yourself or using Fulfillment by Amazon - FBA). Kindle Direct Publishing is for selling ebooks and paperbacks, not general physical products. Amazon Brand Registry helps brand owners protect their intellectual property and manage their brand on Amazon, but it's not a selling platform itself. Merch by Amazon is specifically for selling custom-designed apparel and other print-on-demand products, which is the opposite of what the question asks.