Problems bookmarking a specific item

I am having the same problem with bookmarking/sharing specific items on my website.

It would appear that the permalink for a post ends at http://mysite.com/page (in my case http://www.spare-desks.com/show-ad ) when I actually want users to be able to bookmark/share or save the precise item as follows http://www.spare-desks.com/show-ad/?id=16

Please try to use the bookmark/share icons or the Save Ad function to see what I mean, it assumes the url finishes at /show-ad/ and doesn;t include the ?id=16 part

Any help offered is very gratefully received.

Rob