If the salesperson is taking all the risk, is there any need to interview them?
Unless you're willing to give up your own productive time to train anyone who can fog a mirror the answer is no.
The best way to assess risk when hiring a full commission rep is to consider the opportunity cost.
What can you do with your time if you don't hire this person? You could do the work yourself, or you could hire someone with more potential.
There's also a third option. Hire the best choice and do the work yourself. Letting them observe and shadow you means you can get the work done, minimizing the risk of a bad hire.