If the phone isn't jailbroken, then your only solution is to restore/upgrade to the latest firmware. You can use TinyUmbrella to check if it has any SHSH blobs saved by doing the following:
-Connect the iDevice to your computer
-Open up TinyUmbrella -- Click here
-Click "Advanced options"
-In the "Device/Version" drop-down list, select the firmware you want to check for SHSH blobs
-In the "Request From" drop-down list, select Cydia
-Click "Save My SHSH"
Wait a few seconds while it checks...if it comes back as successful then you do have your SHSH blobs saved on file with Cydia for that firmware and now TinyUmbrella has saved a copy on your computer. This means you can upgrade/downgrade/restore to that specific firmware. If it comes back as failed and says something along the lines of "not eligible" than you don't have your SHSH blobs saved for that specific firmware and can't upgrade/downgrade/restore to it...it's too late.
If no SHSH blobs are saved on it and it's not jailbroken then you're screwed. Also, I find it hard to believe that you bought a phone and the seller wouldn't either remove the password lock all together, wouldn't tell you the password, or you have no way to contact him (how did you buy it in the first place than?)...there's no reason to lie on these forums.