I am a business owner - I hire Independent Contractors to do outside sales. Am I legally responsible to provide an itemization of pay to the contractor?

No - you are only required to document your payments to them using 1099-MISC forms. Presumably they have invoiced you for their services, so no need to provisde the contractor with details from his own invoice.

The contractor should be giving you a bill for their services that you pay.

In situations like commission agents, often they are given a itemsed statement of the payment.

So what type of worker is this, what type of services is being done for you.