There are a few online B2B marketplaces in the US to consider and which one you go with will depend on your requirements.
Amazon Business: This is a very legitimate option now as Amazon looks to expand its offering away from just a B2C model, with a greater focus on B2B. You can offer unique discounts for businesses and a host of other features to drive your B2B business.
ThomasNet: This is more for a traditional supplier/purchaser business relationship but may be a worthwhile avenue if you sell high volume or recurring use items
Alibaba and eWorldTrade: Both Chinese based business so I am unsure of any implications on trading here from the US however there are a huge number of US businesses looking to buy on here and so they are atill some of the largest B2B platforms.