Why are you the best purchasing manager for us?

The best way of answering this question is to be confident and cite your skills that make you the best candidate for this job. For this you may have to go through the job description in advance. Also, you should make sure that you don't sound over-confident.
You can check with your current team members and see what qualities they think you possess which make you good at your job. A little self-introspection might also be a good idea for preparing the best answer for this question.