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I want to mark the last visible item before paging down. Then I immediately unstar it, so a shortcut to do that would be cool, too.

Any other suggestions for accomplishing this task are appreciated, too. It saves a lot of time searching for where to start scanning titles again, as this point is often not near the top nor in the same place each time.

The problem with starring/unstarring with the mouse: those stars are small targets, and if I miss, I often open the RSS feed item instead. A keyboard shortcut would be much easier/faster.

5

Typing s will toggle the star status of the current item.
There are a lot of other shortcut descriptions at Google Reader Help FAQ, such as the keys j and k to move up & down the individual items. without opening them. I do not know though how to move the selected item (which is what gets starred) by anything more than 1 at a time.
I assume you are trying to scan through a page worth of entries before looking at the next page's worth. You might have to get a program like AutoHotKey to map a set of keystrokes, such as 23 n presses in a row, to simulate moving down a whole page at once.

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