Owning, if one so chooses to own, any form of memorabilia is something I support since that is part of "free market". It's harmless of it's been decommissioned and cannot be weaponized so whether it's owning an old rifle or owning an old tank both are allowable as they are fundamentally just trinkets for history buffs.
This is a very bad idea. Even if the unit has not weaponry potential, it did before, and it can again. All it takes is one mad scientist/engineer to reconstruct anything purchased after market. Let us all just hope some rich billionaire doesn't buy every tank possible and try to start an army.
High specification military equipment remains dangerous even if weapons are removed or disabled. This is largely because it is relatively straight forward to re-attach weapons of whatever kind potentially handing powerful military tools to private individuals. There is no guarantee that these individuals would not eventually used this military equipment for a deadly illegal purpose.
I believe this is a terrible idea and I have no idea what good this would do except to make the government a small amount of money. Think of the liability if someone or a group of people use this equipment to hurt other people. I think the only way it could be done is if the equipment was completely taken apart and sold off individually.
Military equipment should definitely not be sold after market. Even if it is 'de-weaponized,' this type of equipment is designed to be used on a battle field, and should not make it's way into civilian life. The chance that something dangerous could happen is extremely high, and this type of equipment should only be used by trained professionals.