Restore with old firmware and re-jailbreak 3gs

hi there,
planning sell 3gs (at&t) jailbroken (used jailbreakme.com) , unlocked ultrasn0w.
want restore factory settings rid of private data.
far know, can restore same firmware has (4.0) holding down shift key while clicking restore button in itunes. have downloaded original firmware.
after restoring it, go jailbreakme.com jailbreak again.

on right track, or missing something?
how activate it? automatically activate jailbreak?
have at&t sim, it's not active, can still use activate?

5th , 6th digit of sn 37. modem firmware 05.13.04

in advance help.

 

do have shsh blob save doe 4.0? if not, can't restore firmware.

 

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