Who do you build messaging for - product or sales?

Neither.

Messaging is built for prospects and customers.

I don't want to diminish the importance of internal stakeholders, but they don't purchase the products you're trying to sell. My advice to address this concern - conduct customer and prospect research that gives you data that not only validates your messaging, but also gives you the intel and credibility to challenge perspectives that don't resonate with those of your customers and prospects. At the end of the day, you're in a great position to advocate for your customers and I'd say those are the people your sales and product teams truly want to help, as well.