Can the Mexican presidential aircraft be sold? I have read that the next Mexican president is trying to sell the B787 that serves as presidential transport, and that it was already made an offering to Boeing for it.

As far as I know, the VIP/presidential aircraft are more or less custom made, so it's hard to imagine that anyone would buy it (I'm guessing an airline would retrofit the plane for his own configuration and that would be more expensive than buying a new plane).

But perhaps the manufacturer would use it for leasing purposes.

Then, it is viable to sell this presidential aircraft (or any other)? How much can be paid for it (compared to the initial value)?  

Reg: XC-MEX
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.