Can the Mexican presidential aircraft be sold?
Not sure about that specific aircraft, but the Dutch government recently sold its transport (a Fokker 70) for the royal family in anticipation of the arrival of its more capable replacement.

I've seen the aircraft many times before it was sold.

I've also seen President's Eisenhower's personal Beechcraft Twin Bonanza, which has been called the very first dedicated Air Force One, which is currently in a private collection in Canada. Not sure when it last flew, but the owner tries to keep his entire collection airworthy and once in a while takes one of them out.