I also had the same problem with a latitude c640 (xxxxxxx-595B) time ago, and after asking here and there, and found out these are the only options:
1. ask the seller for the password
2. Call DELL and prove you're the owner so that they give you a master password
3. Replace a microchip( something like "24xx16") with an empty one
4. Short a couple legs of this microchip while the laptop it is booting up
I am not allowed to post links here yet, so I cannot help you more
By the way, I will never ever buy another used laptop on ebay since most of the times it will be a stolen one.