Over the past few years, many of the most popular sites on the web have begun to give developers windows into those sites' inner workings. With new computer-based security threats appearing every day, Yahoo!, Google, and eBay have all gone to great lengths to open the door to developers. Where the interfaces for the first two are interesting but tend to be focused on creating simple apps, eBay's Developers Program presents a very powerful and feature-rich toolset focused on creating usable applications. But with that power comes some extra complexity not found in the other two APIs.
Before we dig into the details, let's take a quick look at the purpose of the eBay toolset, and how you get started with it. Whereas the APIs you can use to access the Google and Yahoo! sites seem to be angled toward creating cool, free apps, eBay's API is a rather different beast, focused on creating applications that will, in some way, turn a profit.
Because of this focus, releasing an application that utilizes the eBay API "in the wild" (the live production eBay.com auctions) is a complicated process. Google and Yahoo! are perfectly happy to let you search their data and do with it what you will (to a degree). But because the eBay API allows you to insert and bid on auctions, performing transactions that involve real-world money, a formal certification process has been created.
eBay has created a developer membership program with yearly dues and fee-based technical support, with the higher levels receiving extended usage rights to the production eBay environment. Individuals can get limited free access and 10,000 API calls per month, but any large-scale deployment of an application will require someone to cough up some dough and move up to one of the higher levels.