In order to exclude Amazon and eBay customers from receiving emails, you can just enter something like the following in Settings > Filters > Excluded Email Addresses:
%amazonmarketplace.com, %members.ebay.com
Note that for other countries such as the UK there are more specific domains:
%@marketplace.amazon.co.uk, %amazon.co.uk, %marketplace.amazon.co.uk, %amazon.co.uk, %@members.ebay.co.uk
