This problem may have two reasons:
1. Your product is already presented in the other listings.
There's “Products Presented In Other Listings” functionality available in the Advanced Filter that allows you to show, or hide products presented in other M2E Listings.
Most probably it is set to Not Show, so products which are added to other listings are not shown in the search result.
To figure it out, click on the “Show Advanced Filter” button. In a drop-down list on the right, select “Show” and press Search button. Thus, the needed product will be displayed in the product list.
Read more information about Advanced Product Filter for Magento v1.x and Magento v2.x.
2. The product is not added to the Website associated with Listing Store View.
A website contains one or several Store Views. You need to choose the appropriate Website when adding a new Product. However, if the Store View is not associated with the particular Website, the product will not be displayed in the number of products available to add to the Listing. Thus, you have two solutions:
- Select your Product and using Product Information choose the appropriate Website (Catalog > Manage Products > select product > Product Information > Website for Magento v1.x and Products > Catalog > select Product > Websites for Magento v 2.x).
- Create a new Listing and in the General Settings specify needed Magento Store View ( e.g. Sell on eBay > Listings > Add Listing > Step 1: General Settings > Magento Settings > Magento Store View for Magento v1.x and eBay Integration > Listings > M2E Pro > Add Listing > Step 1. General Settings > Magento Store View for Magento v 2.x). Read more about General setting for Amazon and eBay in M2E Pro.