Stay away from ebay binos. 99+% of them are junk. They may look good in the photos but optically they are junk. Very occasionally you might spot a good deal on a 2nd hand pair, but it's very rare indeed.
Best you will do for up to $100 is the 10x50WP model currently on special at AOE. See
http://www.aoe.com.au/binoculars.html
They are a good size for hand held use and AOE's binos have great coatings which help a lot with astronomy (even their cheapies!). I used to own the 7x50HG and it was outstanding value (and outperformed more expensive brand name binos).
PS. whatever you do, don't get a roof prism bino. In this price range they are hopeless.