Suggesting a sales or marketing employee operate solely on commission w/o equity in a startup isn't a great way to gauge their confidence. It's a good way to ensure good sales people run away.

With a new product (or with an old product repurposed for a new industry), it can be hard to predict the sales cycle or to develop the appropriate pricing strategy. If the salesperson is strictly dependent on commission, they'll do anything to close-- including sell the product to the wrong people or with the wrong expectations. This will reduce the amount of useful feedback you get and waste everyone's time.

Someone recently offered me this deal at a startup I otherwise found very compelling. I didn't take it.