Are you really sure you want to sell it? You just might develop an interest in the culture that belies the lifestyle. You are very lucky to have NG available to you, so I do understand your logic.
I suggest, as many others, leave it plumbed in, with shut off valves. The T-Tube unit is an absolute contradiction in technology with the extremely 'low-mass method' vs the 'adequate thermal mass heat retention method'.
Along with hundreds of forum members, I will suggest that you will never be as comfortable with any other form of heat as you will be with coal.
Please don't fall for the 'sucker punch' of : Why would you go backwards in technology and use a carbon based fuel.......when the NG is SOOOOOOOOO efficient and clean.
Also, as to the value of your unit, I'm thinking that a rebuild will be necessary, and that keeps the value below 3000.00 unless someone absolutely has to have the boiler RIGHT now, and overlooks the wear items.
Good luck in your endeavors, and thanks for sharing with the forum.
PS: The Triangle Tube Solo 110 is a VERY fine unit. It has the best warranty and there are PLENTY of them in my area, all burning Propane.......as we have no NG available.