Which is better for getting best prices? Direct negotiations with the suppliers or online auctions?

I prefer online auctions because the vendors will compete with each other during the time of auction and we can get huge discounts and this is useful to the company.The pharma companies like Hetero and Dr Reddy's ,they started using ariba and within an year they are able to get huge savings on purchasing.I personally saw the E-Auctions and the competitiveness of the vendors and it was helpful.