Angel and demon customers is a marketing concept dividing customers into two groups. Angels customers are profitable, whereas demon customers may actually cost a company more to serve than it makes from them. Demon customers attempt to extract as much value as possible out of the seller. Examples of demon customer buying behaviour:
Buying discounted merchandise designed to boost store traffic, then resell the goods at a profit on eBay.
Buying a product, apply for rebates, return the purchases, then buy them back at returned merchandise discounts.