I go to auction sites and enter "Nikon" into their search engines. That tells me how many Nikon cameras, lenses, and accessories are for sale. I do this because I've spent the majority of my time in the photographic area and purchased the majority of my Nikon equipment via eBay.
Actually, I began it as a joke, but the more I got to thinking about it, the more it made sense. This was a way for me to track the activity on each of the auction sites. You can do the exact same thing with something in which you have an interest.
George's Nikon Index has shot up for eBay in the past few days, apparently due to the advertising (I haven't seen the advertising). Yahoo is in second place on the index and is far behind.
The index will let me know if Yahoo (eBay's only competition) starts to catch up or falls further behind. I'll check the sites and post the results tonight (I last posted them a couple of days ago).