There are certainly web based ones that don't tie you to a platform.
I mean perhaps you have a reason for wanting your own version, but the reality is that this had to be tied to the data on Ebay. That's the same no matter what platform you're on. Specifically designing a windows app is limiting when a say Java program could do this work on any platform. Tie that to a web interface and you can use it from anywhere assuming you have a server running.
... which brings us back around to the fact that these exist already.
I'm intrigued... it doesn't seem like it would be too hard. But more information is definitely required. I think you'll be suprised by how much a custom piece of software like that will cost.