SELLERS: Selling popular products that just don't seem to sell for what you expect? Use eBay's MARKETPLACE RESEARCH. This is a subscription-based database which you can get for as cheap as $2.99 for 2 full day's worth of access to 60 days worth of data or $9.99 for a month's worth of access to the same information!
I, myself first purchased marketplace research to find out average selling prices on some of the specific product lines that I sell. I found out that the average selling price of that particular item which was far above what any of mine had ever sold for but I didn't know why. After digging deeper, the research showed me that there weren't many people bidding on the items (1-2 bids on average), which means in order to get the higher price which the customers are willing to pay, I needed to up my starting bid...after all, the only way the sale price goes up on an auction is if there are other people bidding to get it up that high. But when you're in the situation that I was in--where there are few bidders--you, as the seller, need to take a different approach and raise your starting bid in order to make the most out of your listing.
This is just one example of the many ways Marketplace Research can be a great tool in order to increase your sales and profit margins. As an eBay PowerSeller, I highly recommend it--even if you just purchase 2 days worth of access--because every dollar you spend is well worth it if it will increase your sales over all....really a small price to pay for such a great selling resource.
For more on Marketplace Research or to purchase a subscription, go to: http://pages.ebay.com/marketplace_research/detailed-comparison.html