Allowing both Amazon and other merchants causes some Amazon products to be hidden

I notice this too.. even though we get a drop down to select the seller to buy from, it only ever has one entry.

I haven't had time to look into the api response yet to see if there is more data available, or if it's an api limitation.

Here's the relevant excerpt from the API documentation. It appears that just over a year ago, the API underwent some pretty significant changes, which simplify the results, but in doing so reduce some functionality that was available previously.

Product Advertising API Change Details

The following changes will take effect on 11/1/2011:

Associate Tag Parameter: Every request made to the API should include a valid Associate Tag. Any request that does not contain a valid Associate Tag will be rejected with an appropriate error message. For details on the Associate Tag parameter, please refer to our Developer guide.

Seller Operations: The SellerLookup, SellerListingLookup and SellerListingSearch operations will be deprecated. All requests for these operations will be rejected with a corresponding error message.

ItemPage Parameter: For the ItemSearch operation, the ItemPage parameter will have a maximum value of 10 instead of 400. The limit for the “All” search index will continue to be 5 pages. A new element “MoreSearchResults” will be added to ItemSearch responses, containing a link to the Amazon website where your customers can see additional search results beyond the ItemPage limit. All requests for ItemPage greater than 10 will be rejected with an appropriate error message.

Merchants search index: The “Merchants” search index will be deprecated. All requests for this search index will be rejected with an appropriate error message.

ItemAttributes Response group: Selected ItemAttributes that are not relevant to the broader product advertising use case will be deprecated, for the new list of supported attributes, please see the new WSDL.

Offers: The selection of offers is simplified and limited to the most frequently used use cases. You will no longer need to guess which combination of Merchant ID and Condition will return the most relevant offer or make multiple trial-and-error calls.

• The OfferPage parameter is no longer supported and will be ignored.
• A new element “MoreOffers” will be added to the Offers response group, containing a link to the offers page on Amazon.
• Select elements from the Offers response group are being removed:
• Merchant ID, Merchant Location
• Total Feedback Pages, Total Feedback, Average Rating
• Exchange ID
• Quantity
• Sub Condition, Condition Note, Will Ship Expedited/International
• Quantity Restriction, Sales Restriction
• ISPU Store Address, Store Hours
• IsFulfilledByAmazon, IsMAPViolated, MAP Policy
• Please refer to our Developer guide for details on the expected behavior of the API for various input parameters for the Offers response group.

I think I am experiencing similar issues. In my case some items are ONLY available from 3rd party sellers on Amazon, yet the add-to-cart form won't show. It just says "This item is not available from the configured sellers." I have tried toggling "Search Amazon or other merchants" to Amazon as well as All, to no effect.

Tried both 7.x-1.1 and the dev. Don't know what to make if it just yet.
Also, FWIW, I could find no "Add to cart" option in the panel that comes with this module. I ended up adding some PHP code instead:

<?php
$asin = arg(3);
        module_load_include('inc', 'amazon_store', 'amazon_store.pages');
        $form = drupal_get_form('amazon_store_addcart_form',$asin);
        print drupal_render($form);
?>

.. but that is probably a separate issue.