I like where this is going. Too many times there are great products out there that get little recognition because they don't go about marketing and selling properly. I could see this being great if there were some newer companies having sales issues and learning from sales people. Coming from a sales background, one of the hardest things is understanding who might be having the problem you are trying to solve and then going after them. There are plenty of very successful SaaS companies that didn't have a super viral launch but once they figured out their customer went and crushed it. (b2b applications come to mind)

Seems a bit beside the point. I mean, when you go to a hackathon, do you spend all your time telling each other how you hack and coming up with new ways to hack, or do you actually hack?

If there hypothetically were regular sellathons like you describe, I'd probably rather skip most of them and spend the time getting real practice selling. There already are conferences that fill the niche you seem to be thinking of.