I thought I'd share a site I came across recently. Bidvoy searches auctions to show you the average price a product has been going for as well as the trend, so you can see if prices are going up or down, and shortly ending auctions.
If your search has been involved in enough auctions (unlike the Cisco AP above, but like the Nexus 5 below) then the site also tells you the best time to buy and the best time to sell.
What really drew me to the site was how good it looked and functioned on mobile. I think I will be checking this when using ebay in the future.