The webtrends.com website tells what they do - Mobile Analytics | Webtrends
They are gathering data from your phone regarding app usage, web sites visited, and so on, and sending it back to RIM or eBay or other parties. They use it to send targeted advertising among other things. As to why RIM is using it - that would be a question for RIM and/or eBay.
Check the modules of the eBay app and see if there is a module for webtrends. If so you might be able to deny it premissions. When I was using Ubersocial for Twitter, I noticed it had a module called Flurry, which is similar to webtrends. I was able to deny all permissions for the Flurry module and the app still worked, although it would nag for permissions for Flurry.
There are probably a lot more apps using the likes of webtrends and flurry than we know about as mobile advertising and marketing are a hot trend now.