'building on the product' in this sentence means developing or expanding the already existing product. They want to expand their business, without abandoning the original product.
auditing arm selling the consultancy arm to clients.
Selling here means encouraging to use.
The auditing arm (of the mother company) selling the consultancy arm (of the mother company) to clients.
The clients could look for a different consultancy firm. The auditing arms tries to get them to use the consultancy services of their mother company, so that the business stays with Pricewaterhouse Coopers.