According to an article I read called, “The vendor selection process, How to select a vendor formally?”, there are a set of questions you must answer as a buyer:

1. What products do you require?
2. What quality levels are you seeking?
3.What stock levels do you require?
4. How often do you require delivery?
5.What price points are you seeking?

Once these questions are answered then find vendors which meet your demands. Always negotiate prices and not just take the listed value.Once a vendor is selected create a master agreement to layout the delivery times etc.

Reference:
http://www.purchasing-procurement-center.com/vendor-selection-process.html