Bargain shoppers are shoppers who hunt for bargains. i.e. to buy items at the best prices. They typically are people who look at multiple sources of purchase before buying the one with the least price.
More here : http://www.thefreedictionary.com/bargain+hunter
Another meaning of bargain shoppers are persons who are so attracted to bargains that they would buy things they do not need just to get a discount or a free item.
More here : http://www.urbandictionary.com/define.php?term=bargain%20shopper