I just came across this list from David Ogilvy on how to write. The final two points caught my attention and capture the difference between marketing and sales beautifully.
In my experience, the best sales people are continually talking, whether that be face-to face or over some device, they are rarely quiet. Similarly, some of the best marketers are experts at communicating through other media, especially writing.
The simple sentence in point 10, especially after nine, (which in current marketing speak would be "include a CTA") makes the point perfectly, if you want ACTION, writing is the wrong medium.
If you want Sales, talk; preferably face-to-face.
9. Before you send your letter or your memo, make sure it is crystal-clear what you want the recipient to do. 10. If you want ACTION, don't write. Go and tell the guy what you want."