Software developers can use a major site’s Application Programmer’s Interface (API) to provide additional services to users. For example, Auctiva provides listing and photo hosting services to sellers based on eBay’s API. The API is defined by a set of commands a developer can use to perform actions such as AddItem or GetSellerPreferences.
eBay used to charge 3rd party solutions providers for each API use or API call but fees were dissolved in 2005. Auctiva went free at that time. Affiliate programs became more lucrative which is great incentive for solutions providers to drive traffic to eBay by making their services free.
Flash forward to today. Developers have to maintain a positive relationship with their mothership site. It’s a deathwish to do otherwise because you will get cut off from the API. The eSeller Street traffic stats page has changed due to a dispute on Amazon’s API usage. Alexaholic is a 3rd party solution based on Alexa’s API. Amazon owns Alexa – See the very interesting story here.
The major sites with APIs are Amazon, eBay, Yahoo, Google (maps) and YouTube. Take a look at Trulia.com or zillow.com I’m taking a little vacation from eBay to work on a little app for Flickr (owned by Yahoo). It’s called Wurled.com.
More reading: Traffic is shifting from eBay to social networking sites like MySpace and YouTube – See top 500 sites. eBay is a fairly old site at 12 years old.