How-To Example Code
Ebay API
The Ebay API is absolutely fantastic, enabling you to include products relevant to your web portal/site quite easily. But it's not just limited to that, you can query all manner of data from their system and even submit your own listings right onto their site. Apparently 40% of all listings on Ebay are done through API's rather than on the actual site itself.
It's quite simple to get started - All you have to do is signup as a developer and then generate some Auth Keys for your site, they then provide you with some test documents which clearly illustrate where these Keys should be entered ( /common/variables.php).
Apparently the API once cost money, but around about 2005 they made it open and free, but there is a 5000 requests-per-day limit, which is quite a lot really, since a single query can yield hundreds of results.
We use the Ebay API for http://www.marinetrader.com.au to pull in boats every hour and we've also used it on http://www.chipretro.com/>.
For some strange reason, unless im mistaken, is that you have to query a particular site for listings, rather than there being a contiguous data-scape for a user to query. It may be by design, as for Marinetrader.com.au we only tend to seek Australia's site since its for Australian Boating, however for ChipRetro.com we want US as well as UK listings, as UK has a lot more obscure regional retro gaming products, such as Zzap Magazine for instance.