Just a word of caution and not to cause alarm, but there is often talk about the issues associated with lack of use as well as over use. The issues seem to be that seals, rubber components etc deteriorate when not being moved, flexed etc and then begin to fail prematurely when it starts to get used again.
Given that the rig has hardly moved over 15 years, you might want to budget some funds for early repairs, especially on the 'house' part of the rig.
Also, why did he buy 6 tires in addition to the 2 that the previous owner mounted on the front? It doesn't have a tag axle.
Also, he just purchased it from auction for resale and yet writes his description as if he was buying it for himself "I knew I could never afford an $800k new one"
Caveat Emptor - you are dealing with a used car salesman.