Hello, I have a old mac mini and I purchased a previously used newer mac mini to transfer my files to, but it is locked asking for the password. I contacted the seller and he had cleared all of his information and has been trying to help me get in.

Does the lock screen show a Padlock? If so, then it is impossible to remove the firmware lock without providing Apple valid proof of purchase.

https://support.apple.com/kb/HT204455

Or is the computer booting to a login screen? If the computer is booting to a normal login screen then you should perform a clean install of macOS by first using Disk Utility to erase the drive before reinstalling macOS. The photo of the box shows a serial number for a Mac Mini (early 2006). If this serial number is for the "new" Mini, then you will need a retail copy of OSX 10.6 Snow Leopard DVD (one with a black label) in order to reinstall macOS.

Here is what the previous owner should have done before selling the computer (not sure how much of this really applies to a 2006 computer, but erasing & reinstalling OSX does apply even though the exact instructions for erasing the drive may not be correct for 10.6 Snow Leopard):

https://support.apple.com/en-us/HT201065