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If you use a server running IIS as an example it's very easy to redirect from the www. subdomain to the root domain or vice versa. You should probably redirect to the version that currently holds the most authority and rank. The MozBar can help you establish which one this is.

You should read this http://www.google.com/support/webmasters/bin/answer.py?answer=139066#301 Google seem to prefer 301 re-directs where possible, but in your case it may be much more straight forward to use the canonical tag. It was created pretty much for times where you have variations on a product (such as different colour shoes but otherwise identical, each with a separate page)

I would recommend taking a look at slash vs non slash pages and seeing how many non slash pages on your site are listed as 404'ing. Look at your 404'd pages in Google Webmaster Tools. If this happened to one URL chances it happened to many across your site and if so this is a big issue but can be resolved very easily by appending the slash to the end of all URLs. Some code might have broken or been improperly or accidentally changed by a webmaster, because if the slash URL version was originally there then that used to be the standard and something technical caused it to go away. This could have broken if you changed from Microsoft to Linux or vice-versa - the .htaccess file or modrewrite would stop working if that was the case.

Okay take this with a pinch of salt because this is quite old information and things do change quickly in this business. Yahoo used to value pages over domains, so if you had the 'best' page on a subject that is what would count, the information on the domain itself was largely irrelevant, and this may still be the case. Google has a more holistic approach, considering the domain as a whole. This may be what is causing the difference. As Andy says they all have their own algorithms and ways of judging a site, and this is just a symptom of that. It's not really a reason to worry though Google is still the largest player (certainly in the West and especially the EU) and exposure on there will likely help push your rankings on the other search engines up as well over time. Lets face it, if you're doing well in Google you're ahead of the game, you've done well. For the time being just keep doing what you're doing.

Yes! Look at how items can be sorted (i.e. by price, etc... so people can see the cheapest items first). Then think how the products you're feeding in might be sorted by your target audience and adjust accordingly so your stuff comes up first in the sorted results

Anybody else out there?  I too have found these tools to be unreliable....

Thanks - Ill go with the TAG.

Richard, Are you saying I should redirect to pages like this /name/ vs /name

I'm not sure they dont matter, i would think that .pdf, .doc and. xps for example would be less usefull than a htm, asp, aspx. but David is correct, you are better of not haveing a file name. I develop using Microsoft Asp.MVC, there are no file names in MVC i believe seomoz is also written in MVC, cant remeber what tipped me off to that maybe some one from SEOMoz can confirm MVC also leaves very clean code, no postbacks or viewstate to clog up the code

Did you get your site redirected to www? And have you noticed any change in Yahoo?