I have recently bought an iMac but if I want to boot from disc it gives me a Lock screen and I don't know the password

I Bought recently an iMac A1311 when i got it the hdd was broken... so i replaced it with a new one but now if i boot my mac up ik gives me a blinking question mark so I tried command -s on the startup but then it gives me a Lock and i dont know the password neither the reseller

You have a firmware password reset.
The iMac A1311 is (if I'm interpreting info correctly) an early 2009 model.   So this should work:
Shut down the computer and remove all the RAM.
Try booting (you won't be able to even get to the point you were at before, without RAM)
Reinstall the RAM
Turn on the computer and hold down the command-option-p-r key combination before the startup chime sounds.  Continue to hold them until the computer restarts on its own a second time, then release.
The computer should now boot normally.
If that doesn't work...then you'll need a genius bar appointment, for which you will need proof f ownership of your iMac.

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