If you already have Amazon Magento Extension, the other part of the advanced solution is the Amazon Data Extraction Engine, which was designed to sync products over HTML.
Two years ago Amazon started to shrink the usage of the API. If Amazon Magento Extension was able to handle import/sync using HTML parsing itself two years ago, now we need to have a separate tool implemented in the Amazon Data Extraction Engine as a stand-alone project, which is not connected with Magento and does not use API to work.
What is the Solution? Connected Magento + Engine
You keep your inventory in Magneto, use Amazon Magento Extension to find new products, the Engine keeps stock and price updated. Right now, the Engine has a simple product search function. From this point, Magento might be used without Amazon Magento Extension.
This solution allows you to scale inventory size and does not rely on the API
Now we have clients who use the Engine interface to manage products. Magento just connects the inventory to Seller Central, and the Engine manages products on Magento: create/modify/delete automatically. If you remove products from Engine they are automatically removed from Magento.
In order to have the Engine working with 100k-300k products, you need to have a dedicated server with a rotating proxy account. That monthly price will be around $150/mo. If you need more products, we also can discuss this.
After installation and setup, which is included in the package, we also can handle a server management.