Why they let You resell is because it benefits them. They create
hundreds of digital products and some do not allow You to
change the links inside, which are their own affiliate links so by
You and thousands of others promoting the ebook or whatever
their links are going viral.
Plus if You can change the links they are happy just getting the
price of the licence from You and thousands of others, plus by
getting You to buy, they got the bonus of getting Your email
address, so they can continue to market to You recurring.
The reason You would make sales is someone would buy from
Your site before another one is because You attracted them
better/got in front of them sooner, had a better promo and sales
copy, squeeze page, priced Your product more cleverly offered
extras, had a free offer first of all to get the email address etc.
Your sister is right, You will always be better off selling Your
own product if its good enough.
The simple fact remains though, that it does not really matter
what You are selling, You can sell anything, it All comes
down to knowing how to attract Qualified/TargetedTraffic/
Visitors/Buyers. If You cannot do that then all is lost anyway.
Everybody needs Top Quality Economical Internet/Affiliate
Marketing training to learn the proper up to date strategies for
success Online. Social Media Marketing such as FaceBook
etc are the now and the future of selling and profiting Online.
Study and prosper.
I wish for You all that You wish for Yourself,
My source is mainly My many years of "been there done that" and life experience"