Got to take into consideration this amp apparently is immaculate in showroom condition......that is worth a big premium imho.
Last year I sold a close to mint un-modded 82 JCM800 White Anv 2204 full stack for $3,500. Had it listed here for quite awhile with no takers, ended up selling local. It was a hell of a deal.
Are the tubes 6650s or EL34s? Without researching Marshall history I know that there were times when Canadians got EL34s while the US got 6550s. I’ve had both and feel that the true Marshall classic sound comes from EL34s and not 6550s. If it’s EL34s that would be an awesome sounding amp, but it may need new capacitors even if it hasn’t been played because they will age with time.