eBay is an online auction site whose core function is to provide a platform for sellers to sell items and for buyers to buy those items.

Here are some of the core functions of eBay:

1. Auctions: The original core function of eBay was auctions. Sellers can set a starting price and auction time, and then wait for buyers to bid. After the auction ends, the highest bidder wins the item.

2. Instant Buy: In addition to auctions, eBay also offers an instant buy feature. Buyers can buy items at a fixed price without waiting for the auction to end.

3. Search: eBay provides a powerful search function that allows users to easily find the items they want.

4. Categories: eBay divides items into multiple categories for users to browse and search.

5. Rating system: eBay's rating system allows buyers and sellers to rate each other, so that users can better understand each other's credit and reliability.

6. Payment System: eBay provides a secure online payment system that allows buyers to easily pay for items they purchase.

7. Logistics: eBay provides a logistics tracking function that allows users to check the logistics status of their items.

8. Customer Service: eBay provides customer service support to help users solve problems they may encounter during the shopping process.