You'll have a hard time finding a used camera for $100. If you do, it will most likely be 2 or 3 MP's. I have a Sony Cybershot P73 that I like. It's only 3MP, but I can get 8X10 prints from it that look good.
With any digital, make sure you have OPTICAL zoom. Digital zoom sucks ass. You can quite often find cameras that have both, which is fine.
Kits Camera has a Nikon Coolpix L4 which is 4MP for $150. The library district I work for just bought a P2 5MP with Wireless for just under $200 and it shoots nice images.