I've been through half a dozen startups. They each go through several Sales teams before they're done - try some guys, sorry not working, get some more. What does that tell you?
In the same way as a layman hiring a technical guy might not know what to look out for ("This guy knows ActionScript and says that's what we should build our web app in! He's a great developer!"), a layman hiring sales or marketing people is going to get shafted.
Good sales people are at least as scarce as good engineers. The best ones can sell anything, so why would they bother selling subscriptions to your web app for a $20 commission when they can sell oil refinery equipment for a $200k commission? They aren't interested in challenges or doing new and innovative things like engineers are. They're economically driven.
Sales people have the greatest jobs really! They either perform or they don't.. Its easy to measure and easier to take action.
What if the person had a compensation package which paid really well for selling a product which could be sold by over promising? Their W2 would look awesome but the company would be screwed. I would be most cautious of business people leaving a job with a strong w2. Also it is important to see relative performance. What if they are making 100k and their coworkers are bringing home 500k? Are they still good then?