They want to show the listings that will most likely satisfy the user. Search engines like Google and Cassini exist to help match people with the things they are searching for. Google is the most famous search engine, but eBay also has its own search engine named Cassini. SEO, which stands for search engine optimization, is the practice of making your listings more appealing to the algorithms that determine which pages show up in search results. Before your product listings can sell on eBay, people need to be able to find them.