The sentence makes no sense.
It's not a very good sentence. It sounds to me like there is a producer. He sells it to his customer. That customer then modifies or bundles it and sells it to the customer's customers.
I make a computer and sell it to you. You take that computer, load software on it and sell it with a printer as a package to your customer.
I make and sell you a transmission. You put the transmission into a car and sell it to your customer.
I'm not a teacher. I speak American English. I've tutored writing at the University of Southern Maine and have done a good deal of copy editing and writing, occasionally for publication.
Yes, you are right. It is taken from a website. So we'd all agree that the customer is the one who modifies it and then sells it to the public.
Yes, but it's not a good definition since it uses 'customers' to mean two different groups of people. It might be OK with "to their customers" on the end.
Or who then modify....