If you're thinking about buying anything on eBay, you might want to do a little research first. Checking their closed item search will give you a pretty good idea of what you should (or should not) be paying for the item you want. That is assuming you're not looking for a one of a kind item.
Most items like iPods or macbook pro laptops are commodities and the pricing is therefore somewhat uniform. But scrolling thru that long list of (up to) 200 items with sold/unsold items makes it hard for you to get an idea of what you should be paying. Sure, you could try to remember the cheapest item you saw, or the 2 or three that seemed more reasonable. What if there was a better way to get some research on those items? Now there is! Just add the bookmarklet on this page to your toolbar and you can easily see what the run down on those listings are.
When you search for your item, try to get as specific as possible using negative keywords (-notThisKeyWord), category filtering, price range, and product specifications. This will help to make removing trash items much easier. Don't make it too specific either because then you may leave out an item where someone listed it wrong (which usually results in a bargain buy). Once you're happy with the closed item results, just click the bookmarklet in your toolbar and it will take you to the research tool page. Here is a screenshot of what it looks like:
There are a few things you can do now. The three top boxes show you summary information. The first box shows the breakdown of number of sold vs unsold items and how many items were found. Currently the tool only takes the first page of data from your search, but I am planning to add a feature to allow multi-pages. The second box shows the average, minimum, and maximum sale price for sold items along with the standard deviation in the sample set. The third box is the same as the second except for unsold items. The minimum and maximum values are clickable and will filter the data table by that value. So if the minimum is $9.99, clicking it will filter out any prices not containing 9.99. You can sort on the price column to see the lowest/highest prices. Clicking on a row will select that row and you can remove it from the data set by then clicking "Delete Selected Row". You can also select multiple rows by holding down the SHIFT key for your second click.
Using the standard normal distribution table, we can remove any prices outside of 1.645 standard deviations away. That removes the 10% of samples that are outliers both low and high. You can do this in one step by clicking the "90% Sold Range" or "90% Unsold Range".
The above is an analysis of user submitted closed items from eBay. The raw data being used is shown below in the data table. There are some easy ways to clean up the data. The automated method will assume a normalized distribution and allow you to remove outliers. Clicking on the "90% Sold Range" button will remove items less/more than 1.645 standard deviations for sold items. "90% Unsold Range" will do the same for unsold items. Manual pruning of the raw data can be done by simply selecting data rows and clicking the "Delete Selected Rows" button. Clicking on the min/max prices will display just those products that contain that price, allowing you the decision of removing those items or not. One last way to edit the data is by clicking on the Price column in a given row. Sometimes items are sold in multiple quantities and you can edit the price to account for that.