A ton of features are available through eBay Web Services, and more are added every two weeks. Right now much of the focus is on the new schema and the most eagerly awaited new feature: Want It Now APIs. My favorite recently added feature is the new AddToWatchList call. This call allows you to add one or more items to your My eBay watch list. Here is an example call in the New XML API format:
<?xml version="1.0" encoding="utf-8"?>
Thats it! (Here is a link to the item I added to my watch list, in case you care). One cool thing is that you can include multiple ItemIDs in one call, so you can add multiple items to your watch list all at once. The call returns a count of the number of items in your list, which is nice. There are also two corresponding calls to use: RemoveFromWatchList and SetUserNotes (which lets you set the yellow sticky notes you can attach to items tracked in My eBay). For more information on the call, search for "AddToWatchList" in the documentation.
One cool use of this call I'd like to see is a subscription service that adds one cool listing to my watch list per day, based on criteria that I specify. I would also like to add this call to the eBay - TiVo application that is hosted on the eBay Community Codebase. Right now that app lets you search for and purchase an item, but it would be cool for it to also let you also add items to your watch list in case you aren't quite ready to purchase. I'm busy for the next month or two working on a super secret project, but if someone wants to step up to the plate and update the TiVo app with this functionality (the app is open source, after all), you will be a hero.