Tracking sales price of physical silver (and gold) on ebay

I'm building a charting tool for a website to track sales of Silver Eagles, Gold Eagles and Krugerrands on ebay. I'm hoping to have a tool that's useful for comparing the average sale price against spot. Question for you Turdites - what would you find more useful/interesting - previous day sales only or a moving average over 3 or 7 days or so? Also, is there another coin (Maples?) or product (that can be found via searches with a high degree of false positives) that you would like to see in such a chart?