I want to sell digital products (windows, antivirus etc.) Legally without any problems with EBAY

I need to find those products having licence key that I can provide to customers.( the licence key must be for multiple activations )Many sellers on ebay sell windows and other digital traded mark products and their listing is not removed by ebay. I have done a researc and the most of them claim to have licence given by the manufactures ( microsoft, karpefsky, norton etc.) I need to have information about how to manage any situation regardin customers claiming that the product I am selling is not original and also how to manage this situation with ebay it self if the request me to delete the [login to view URL] over I need information on how to sent all those digital goods to customers automatically ( what application can I use and how can I set this up). send me a message and tell me how you will help me in this