In Inkscape, when I use the "Trace Bitmap" function, how can I avoid ending up with the outline of the image when I want to get a single line sketch of the image?

As far as I know in my experience with Inkscape, I don't think there's a way to turn a Bitmap into the kind of single line Vector image you're looking for. The closest thing you can get would be to trace the image with the Trace Bitmap function, edge detection set to zero will find all the outlines for you if you need it to. After that if you select it and then choose Path > Simplify it will reduce the number of Vector nodes significantly which should go some way to helping you make edits. Other than that, I guess you're left with tracing it manually to get a single line vector. If there *is* a way to do it automatically, I'd like to know too. :-)

You need to do a "centerline trace". I don't believe Inkscape has that feature.Here are two free, online options (remember to select "centerline"):http://www.roitsystems.com/cgi-bin/autotrace/tracer.plhttp://online.rapidresizer.com/tracer.phpApparently, the best programs for centerline trace are Adobe Illustrator or Corel Draw. There are other software options as well, but I haven't tried any of them.

What you are after is called "center line trace" Inkscape does not have that function (yet). Some similar results can be obtained from a limited set of images by selecting one of the resultant scanned paths ( if you have multiple colors for example you break apart the group and select one of the color's paths) and set the stroke to something and the fill to empty. Perhaps do this for a number of the paths. Break apart the paths and remove what you do not want. Again this will only make nice looking "drawings" for a very limited set of images that have the appropriate qualities and it requires a lot of "hands-on" work.