eBay expects sellers to consistently provide service that results in a high level of buyer satisfaction.

When a seller lists an item on eBay and a buyer bids on or buys it, the seller and buyer enter into a contract that both members are expected to honor. For sellers, this contract includes both formal requirements, as well as informal "common-sense" obligations to provide good service to their buyers. 

If the seller doesn't meet these requirements, it leads to a bad experience for the buyer and may result in negative or neutral Feedback or low detailed seller ratings (DSRs) for the seller.

To learn more:
eBay US - http://pages.ebay.com/help/policies/seller-non-performance.html
eBay UK - http://pages.ebay.co.uk/help/policies/seller-non-performance.html
eBay HK - http://pages.ebay.com.hk/help/policies/seller-non-performance.html