Hugh Williams (disclaimer: my boss) has been posting a series on ranking. Part 3 talked about testing new ranking formulas including the unfortunate fact that some people may see their items get lower rankings than before. This negative stood out a little for me so I thought I would comment on it as to why it may not be as bad as it sounds.
It may be obvious, but ultimately the goal of eBay is to put things in front of people that want to buy them. Putting items higher in a ranked list for someone not interested in the item is not better for the seller or eBay. This can make it hard for a seller to really work out how to get their item higher in listings. If they search for the item to see how it looks immediately after listing it, this may not be a good measure of how well it ranks. (I am not saying its not worth doing, but it has flaws.)
For example, for an auction, a lot of people wait until near the end of the auction before bidding. For myself, I did this the other day simply because I wanted something quickly. I did not want to wait 3 days before working out if I won it or not. If I lost the auction I wanted to move on immediately and bid on something else. If eBay used the duration before closing time of the auction in the ranking forumula (I am going to avoid any commitments about what eBay does or does not do to avoid need for the legal department to review this post!!!), then doing a search just after an item is listed on eBay will not tell you how it will rank just before the auction closes.
Another aspect is of course it is important for eBay and sellers to increase the number of people who find eBay useful to find things they want to buy. The more buyers find eBay useful, the more they will come back. The more they will come back, the bigger the potential market for sellers.
So even though one change in ranking might for a specific case lower the position of a product in a rank result, overall sellers may still be at an advantage if eBay can (1) keep increasing the total number of sales (and the customer base size) and (2) put the item only in front of people likely to buy it.
Disclaimer: These are my personal opinions, not necessarily those of eBay (etc etc) – the above does not necessarily reflect what eBay does internally now or in the future. I just wanted to point out it is complex to evaluate.