Cool idea! Here's my feedback: Feels like there is too much on the page that doesn't do anything. Since this is essentially a search box with a button and a result, I would eliminate almost everything on the page and focus all attention on that search box and the results. Also you should show something if the search doesn't come up with an average, perhaps the few listings that did come up. Better yet, show the eBay search results no matter what, so I can edit my search if it looks like the wrong items are being averaged.

There are a lot of ebay market research tools out there. You will need to stand out either by pricing or by feature. Good luck.