Here is what you can do:

1. First reset the "Device Lock" code to 1234 from Menu > Settings > Device Lock
2. Flash your N900's FIASCO/rootfs image from here, without the -R parameter (-R - Restarts after flashing)
3. Flash your N900's eMMC from here.

If you do not want your N900 to boot after flashing everything, do not use the -R in step 2. By doing this, the person who gets the phone from you will be able to select the date, time, region etc and he might feel good about it

Note: As you want it to be in factory condition, the third step will take care of deleting every data that you have on your N900.

Hope this helps.

On a completely different note, why do you really want to give your N900 off?