Bring it back to the people who had the yard sale and ask them to enter the password to "unlock the laptop". I assume it is the BIOS password which you should write down. If the password used is correct, then you should be able to go into the BIOS and disable the password.
If they do not remember the password, then you should ask for a refund because obvious a laptop with an unknown password is simply a technological paper weight.
Do not bother contacting Dell support because they will only help you if you can prove you are the original owner.