Follow the below mentioned steps to install Import from eBay app to your BigCommerce store.
Step 1: Login to your BigCommerce store admin and click on the left menu: Apps -> Marketplace.
Step 2: Now click on the button BIGCOMMERCE.COM/APPS as highlighted in the above screenshot.
Step 3: Type the App name Import from eBay in the search box at BigCommerce app store. Click on the Import from eBay app as shown in the below screenshot.
Step 4: Click on the Install button.
Step 5: It will ask for the authorization to install the app. Click on Confirm to complete installation.
Connect your eBay seller account by following the instructions at: How to link eBay Seller Account