I know this question is someway similar to this one where they reccomend using 404, but after reading this other one where they suggest to use 301 when changing site urls (in the specific case was due to redesign/refactoring) I get a bit of confused and I hope someone could clarify for this specific example:

1. Let's say I have an ecommerce site,

2. let's also say the final user inserted some interesting items in the site and the ecommerce webapp created the item pages at the urls: http://...?id=20, http://...?id=30 etc.

3. Now let's say some of these interesting items got many external links toward them from many other sites because some people found those items very interesting and linked to them.

4. After some years the final user deletes those items, so obviously the pages/urls http://...?id=20, http://...?id=30, etc. now do not exist anymore, but still many pages on the web are linking toward them.

What should the ecommerce site do now, just show a 404 page for those items?

But, I'm confused, wouldn't this loose all the Google PR passed by the external links to the items pages? So isn't it better to use 301 redirect to HOME PAGE that at least passes the PR to the HOME PAGE?

Thanks,

EDIT:

Well, according to answeres the best thing to do so far is to do a 404/410.

In order to make this question more complete, I would like to talk about a special case, just to make sure I understood. properly.

Let's say the user creates those items again (the ones he previously deleted at point 4), maybe he changes a bit their names and description, but they are basically the same items. The webapp has no way to know these new added items were the old items so it obviously create them as new items with new urls http://...?id=100, http://...?id=101, does it makes sense at this point to redirect 301 the old urls to the new ones?

MORE EDIT (It would be VERY IMPORTANT TO UNDERSTAND):

Well according to the clever answers received so far it seems for the special case, explained in my last EDIT, I could use 301, since it's something of not deceptive cause basically the new pages is a replacement for the old page in term of contents. This is basically done to keep the PR passed from external link and also for better user experience.

But beside the user experince, that is discussible (*1), in order to preserve PR from external broken linlks why not just always use 301, In my understanding Google dislikes duplicated contents, but are we sure that 301 redirect to HOME PAGE is seen as duplicated contents for Google?!

Google itself suggests to redircet 301 index.html to document root so if they consider 301 as duplicated contents wouldn't that be considered duplicated contents too?! Why do they suggest it?

Let me provoke you: “why not just add a 301 to HOME PAGE for every not found page?”

(*1) as a user, when I follo a broken url from some external link to some website's page I would stick more on this website if I get redirected to HOME PAGE rather than seeing a 404 page where I would think the webiste does not even exist anymore and maybe I don't even try to go to HOME PAGE of the website.