|"If the seller doesn't know the password then they just might not have been the rightful owner of it."|
In other words, it's likely they stole it, or someone they got it from stole it.
"If everything is legitimate... Dell will happily oblige."
Most brand names will NOT help you remove a bios determined bios access, system access, or hard drive access password unless you can provide enough info to them to prove you're the original owner.
You can't remove the passwords on laptops made in the last ten years or more by removing the cmos battery or clearing the cmos otherwise. The password information is stored on a separate chip other than the bios/cmos chip that cannot have it's user data cleared by removing the power to it. Programs that can be used to clear passwords on older laptops do not work on modern laptops.
The Rules of this web site:
Do not post any questions or answers that infringe on another party's intellectual property. This includes questions asking where to obtain or how to use "warez," "cracks," or anything like that. It also includes posts related to circumventing time limits in shareware software and illegally obtaining software "key codes." Remember, most abandonware is a myth. Copyrights on software last far longer than five years.
Do not post any questions or answers having to deal with hacking or cracking. These include, but are not limited to, entering restricted places on networks or messing up other people's computers.