Holy cow, I wrote a book!
Remember that there are two types of scrollbars.
One is the standalone scrollbar control.
This one has its own window handle, and consequently
can be given focus and all those other fun things
you can do with window handles.
To manipulate them, pass the handle to the scrollbar
control to the appropriate scrollbar function
(SetScrollInfo, for example)
and pass SB_CTL as the nBar parameter to indicate that
you have a scrollbar control.
The other type is the horizontal or vertical scrollbar
(or both) attached to a window by virtue of having
the WS_HSCROLL and/or WS_VSCROLL style.
These are nonclient scrollbars and are not controls.
They are just decorations added to some other window.
You can't give them focus since they aren't windows
in their own right.
To manipulate them, pass the handle to the containing window
to the appropriate scrollbar function
and pass SB_HORZ or SB_VERT as the nBar parameter to indicate that
you want to manipulate the nonclient horizontal or vertical scrollbar.
I'm writing this down since some people seem to miss
the distinction between these two cases.