I have no experience with the Atrix but if its like any other phone if you can get to the bootloader. You should be able to do a factory reset from there, it is usually some combo of the power button and the volume buttons or other hardware buttons. If not then its locked until you can get the unlock pattern. I can't remember what happens if you try to many times, I know it gives you the option to reset the password with the gmail attached but it may also give you the option to factory reset, beyond that IDK.
It also said if I enter it too many times it'll will only verify with a gmail account
I did a power button and Volume up key and got starting rsd protocol support
Its been awhile but if you can get to the recovery you may be able to factory reset, other then that like i said I know very little about it.
The person you traded with should have really factory reset it before giving it to you, but glad to hear you figured it out.