Whether you're going for the free option or plan on spending a little money on this project, your first stop is http://developer.ebay.com. Here you'll find all the information about membership tiers, a sea of documentation, a forum for soliciting peer support (which is all you're going to get at the individual level), and of course downloads for the various SDKs.
The first task is the creation of a developer user ID to access all the tools on the Developers Program site. This developer login must be tied to a production eBay user ID. Additionally, you'll need to create one or more users in the eBay Sandbox, a certification environment for testing eBay applications. Until your application has been fully tested and certified, the Sandbox is all you can access. You'll need to create a set of keys and tokens that your application will use to identify itself and its developer. At the end, you'll have at least three separate user IDs and passwords, along with three keys and two or more lengthy tokens. Good luck if you hope to remember all that.
eBay offers a raw web services–based API, usable from any developer toolset that's compatible with XML, HTTP, or SSL. For those who prefer a little less hands-on approach, there are also a number of SDKs that wrap the base API, enabling easier object-based interaction. SDKs are available for Windows developers using COM or .NET-compatible languages or Windows-based Java developers (with a JAR and associated DLL), and there's a separate toolset for PHP, Perl, and Python developers.
However, while the Microsoft-oriented tools for Visual Studio and .NET developers are comprehensive and reliable, the other SDKs are not up to quite the same level. The Java SDK requires a DLL, so using it will generally entail a developer working under Windows. Beyond that, in testing for this article it was found to be somewhat unreliable, depending on the JDK and Windows version. Likewise, the PHP, Perl, and Python tools were all developed using an older version of the eBay API schema, set to be killed off in June 2006, so applications written using these tools may have short lifespans.
For this reason, we'll stick with the .NET SDK in this article. Code examples here are presented in C#, but the basic concepts and calls are applicable whether you use raw XML or one of the other SDKs.